Methods for oriented growth of nanowires on patterned substrates

ABSTRACT

The present invention is directed to systems and methods for nanowire growth and harvesting. In an embodiment, methods for nanowire growth and doping are provided, including methods for epitaxial oriented nanowire growth using a combination of silicon precursors, as well as us of patterned substrates to grow oriented nanowires. In a further aspect of the invention, methods to improve nanowire quality through the use of sacrificial growth layers are provided. In another aspect of the invention, methods for transferring nanowires from one substrate to another substrate are provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/754,519, filed Dec. 29, 2005, the disclosure of whichis incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Portions of this invention may have been made with United StatesGovernment support under a grant from the National Science Foundation,Grant No. IIP-0620589. As such, the United States Government may havecertain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to nanowires, and more particularly, tonanowire growth and harvesting.

2. Background of the Invention

Nanostructures, and in particular, nanowires have the potential tofacilitate a whole new generation of electronic devices. A majorimpediment to the emergence of this new generation of electronic devicesbased on nanostructures is the ability to effectively grow and harvestnanowires and other nanostructures that have consistent characteristics.Current approaches to grow and harvest nanowires do not facilitate massproduction and do not yield consistent nanowire performancecharacteristics.

What are needed are systems and methods to grow and harvest nanowiresthat have consistent performance characteristics.

SUMMARY OF THE INVENTION

The present invention provides methods for producing nanowires. In oneembodiment, one or more nucleating particles are deposited on asubstrate material. Then, the nucleating particles are heated to a firsttemperature and then contacted with a first precursor gas mixture tocreate a liquid alloy droplet and initiate nanowire growth. The alloydroplet is then heated to a second temperature and then contacted with asecond precursor gas mixture, whereby nanowires are grown at the site ofthe alloy droplet. The substrate material utilized in the processes ofthe present invention may be crystallographic or amorphous. Suitably,the substrate material comprises crystallographic silicon, eitherpolycrystalline or single crystalline. In other embodiments, thesubstrate may be amorphous SiO₂, Si₃N₄, or alumina.

Additional methods for producing nanowires (e.g., Si nanowires) are alsoprovided by the present invention. For example, such methods includedepositing one or more nucleating particles (e.g., a metal catalyst suchas gold nanoparticles) on a substrate material (or nucleatingnanoparticles on a substrate surface (e.g., by heating a gold filmcoating layer on the surface)). The nucleating particles are then heatedto a first temperature at which a first precursor gas decomposes to forma eutectic phase with the nucleating particles and then the nucleatingparticles are contacted with the first precursor gas mixture, whereinthe first precursor gas mixture comprises a first precursor gascomprising at least one atomic species (e.g., Cl) that assists inorienting the growing nanowires (e.g., by etching as described in moredetail below). The nucleating particles are then contacted with a secondprecursor gas mixture after initiation of nanowire growth, wherein thesecond precursor gas mixture includes a precursor gas that decomposes toform a eutectic phase with the nucleating particles at a secondtemperature which is lower than the first temperature, and heating thenucleating particles to the second temperature.

The above method can be reversed such that the process of nanowiregrowth is initiated with a precursor gas at the lower temperature, andthen nanowire growth is continued at a higher temperature using a secondprecursor gas (e.g., a gas having a reactive etchant species to aid innanowire orientation such as chlorine). The first precursor gas utilizedis preferably SiCl₄ or SiH₂Cl₂ which contains Si and Cl atoms upondisassociation at the first temperature. The Si atoms provide fornanowire growth and the Cl atoms allow for growth of the wires in a<111> orientation when grown on a crystallographic substrate as a resultof etching of the native oxide layer on the silicon substrate. Oncenanowire growth has been initiated, a second precursor gas mixtureincluding a precursor gas such as SiH₄ or Si₂H₆ can be introduced whichdecomposes to form a eutectic phase with the nucleating particles at alower temperature than the first precursor gas. The disassociated Siatoms from SiH₄ or Si₂H₆ at the second temperature continue the growthof the Si nanowires. Thus, nanowire growth can continue with the free Siatoms at a lower temperature than that at which nanowire growth isinitiated, e.g., allowing growth of the oriented wires to a desiredlength while minimizing diffusion of the metal catalyst into the growingnanowires.

The substrate material utilized in these methods may be crystallographicor amorphous. Suitably, the substrate material comprisescrystallographic silicon, either polycrystalline or single crystalline.In other embodiments, the substrate may be amorphous SiO₂, Si₃N₄, oralumina

In embodiments where crystalline substrates are utilized, the wiresgrowing on the substrate material can preferably grow in an epitaxialorientation. Nanowires produced according to the processes of thepresent invention grow out of the plane of the substrate material, andare capable of transporting electrical charge.

In certain suitable embodiments of the methods of the present invention,the first temperature to which the nucleating particles is heated ishigher than the second temperature to which the alloy droplet is heated.Suitably, the first temperature is at least about 50° C. higher than thesecond temperature. The nucleating particles used in the practice of thepresent invention will suitably be a metal catalyst and will comprise ametal that reacts with both the first precursor gas mixture and thesecond precursor gas mixture (i.e., decomposed first and secondprecursor gas mixtures) to form a eutectic from which Si mayprecipitate. Suitable metal catalysts comprise Au, Pt, Fe, Ti, Ga, Ni,Sn or In and in certain such embodiments, may be a Au colloid or Aufilm.

The first precursor gas mixture and the second precursor gas mixtureutilized in the processes of the present invention will suitablycomprise SiH₄, Si₂H₆, SiCl₄ or SiH₂Cl₂, and may further comprise B₂H₆,trimethyl boron (TMB), POCl₃ or PH₃ (e.g., as dopant materials).Additional embodiments of the processes of the present invention mayfurther comprise contacting the growing nanowires with one or moreadditional precursor gas mixtures comprising SiH₄, Si₂H₆, SiCl₄ orSiH₂Cl₂ and further comprising B₂H₆, TMB, POCl₃ or PH₃ to grow thenanowires to a desired length. The precursor gases used in the processesof the present invention may also suitably be introduced via plasmaenhanced sputter deposition.

In another embodiment of the present invention, the need for growingnanowires at different temperatures can be avoided by growing thenanowires at lower temperatures (e.g., lower than about 800° C.) usingPlasma Enhanced Sputter Deposition (or Plasma Enhanced Chemical VaporDeposition (PECVD)). In this embodiment, the nucleating particles arecontacted with a precursor gas mixture that preferably includes aprecursor gas comprising a reactive species (e.g., Cl) that aids inorienting the growing nanowires, such as SiCl₄ or SiH₂Cl₂.Alternatively, the precursor gas mixture may include chlorine gas (orplasma) from a separate, independent source that can be provided incombination with one or more of the precursor gases discussed above(e.g., SiH₄, Si₂H₆, SiCl₄ or SiH₂Cl₂). Where the precursor gas mixtureincludes SiCl₄ or SiH₂Cl₂, decomposition of SiCl₄ or SiH₂Cl₂ into Si andCl in the presence of a carrier gas (e.g., H₂, H₂Ar) forms HCl. Asdiscussed in more detail below, this creates a competition betweenetching with HCl and growth from the Si vapor. Chlorine aids in removalof interfacial oxide on Si substrates leading to the oriented nanowiregrowth. The addition of an independent source of chlorine gas has theadvantage of allowing the reactive species of Si and Cl to beindependently controlled in the plasma to enhance or suppress etching asneeded to promote nanowire growth. Sputter deposition can beaccomplished via any method known to the ordinarily skilled artisan, forexample, diode, radio frequency and direct current deposition.

The present invention also provides methods for producing nanowireswhich do not involve metal catalysts, including, for example,hydroxylating a substrate material, contacting the substrate materialwith a first precursor gas mixture, forming one or more nuclei (e.g.,nanoparticles) on a surface of the substrate material, contacting thenuclei with a second precursor gas mixture, and growing nanowires at thesite of the one or more nuclei.

In another aspect of the invention, additional methods for nanowiresynthesis are provided. In an embodiment, a method for nanowiresynthesis includes positioning a granular precursor material at one endof a vessel at one temperature and positioning catalyst particles at anopposite end of the vessel at another temperature. Materials are thentransferred from one end of the vessel to another. A transport agent isreacted with the granular nanowire precursor material to form nanowires.In an alternative embodiment, a similar method is provided to dopenanowires.

In a further aspect of the invention, methods to improve nanowirequality during manufacturing are provided. In particular, a method toreduce surface states from dangling bonds on a nanowire structure isprovided. In an embodiment, the method includes creating a nanowirestructure, depositing a sacrificial layer on the nanowire structure,passivating the nanowire structure with the sacrificial layer, andchemically removing the sacrificial layer to free the nanowires.

In another embodiment, a method for producing a nanowire device isdisclosed that includes providing a substrate having nanowires attachedto a surface in a vertical orientation, depositing a dielectric layer onthe surface of the nanowires, depositing one or more nanowire contactson the nanowires, depositing a material over the nanowires to form ananowire composite, and separating the nanowire composite from thesubstrate.

A series of methods are also provided for harvesting nanowires. In anembodiment, a method includes growing a nanowire with a desired portionand a sacrificial portion. The desired portion has different propertiesthan those of the sacrificial portion. In one example, the sacrificialportion is an alloy and the desired portion is not. In another example,the sacrificial portion is doped differently than the desired portion.Wet etchants are used to differentially remove the sacrificial portionof the nanowires. The example wet etchants etch away the sacrificialportion of the nanowire at a far greater rate than the desired portion.As a result, nanowires can be produced with consistent lengths.

In another embodiment, silicon nanowires are grown on a siliconsubstrate in which the orientation of the silicon in the nanowires isdifferent from the orientation of the silicon in the substrate. Forexample, the atoms in the silicon nanowires can have Miller indices of111, while the silicon atoms in the substrate can have Miller indices of100. As in the previous case, wet etchants can be used to differentiallyremove portions of the silicon substrate to free the nanowires.

In another aspect of the invention, methods for transferring nanowiresfrom one substrate to another substrate are provided. The methods can beused, for example, to transfer nanowires from a nanowire growthsubstrate to a device substrate. In an embodiment, the method includescoating a transfer surface with a non-stick coating, such as TEFLON. Thetransfer surface can then be pressed against nanowires that are affixedto a nanowire growth substrate. The nanowires become stuck to thetransfer surface with the non-stick coating. The transfer surface isthen positioned over a device substrate, and pressure is applied to theback of the transfer surface to release the nanowires onto the devicesubstrate. In alternative embodiments, the transfer surface can bepatterned with a non-stick coating to match areas on a device substratewhere nanowires are to be placed. In a similar alternative embodiment,the non-stick coating can be distributed all over the transfer surface,and pressure can be placed on the backside of the transfer surface in apatterned fashion to release nanowires onto particular areas of a devicesubstrate.

In an additional embodiment, methods for harvesting and transferringnanowires are disclosed that include providing a substrate materialhaving one or more nanowires attached to a top surface. A transfersubstrate is then provided, oriented above the top surface of thesubstrate. Pressure is applied to the transfer substrate, such that thetransfer substrate is brought in contact with the one or more nanowires.One or more of the nanowires are then transferred from the substrate tothe transfer substrate, and the transfer substrate is then removed. Inthis embodiment, the transfer substrate can be a flexible polymer and aprobe can be used to apply pressure. In embodiments the probe may beheated or the substrate may be heated.

In a further embodiment, the present invention provides methods forproducing nanowires on patterned substrates. In suitable embodiments,the methods comprise layering a catalyst-repelling material on asubstrate material to at least partially cover the substrate material.One or more nucleating particles are then deposited on the substratematerial. The nucleating particles are then heated to a firsttemperature and contacted with a first precursor gas mixture to create aliquid alloy droplet to initiate nanowire growth. The alloy droplet isthen heated to a second temperature and the alloy droplet is thencontacted with a second precursor gas mixture, whereby nanowires aregrown at the site of the alloy droplet. Suitably, the substrate materialwill comprise a crystallographic material, such as Si, and thenucleating particles will comprise metallic films or colloids, such asfilms or colloids comprising Au, Al, Pt, Fe, Ti, Ga, Ni, Sn or In.Examples of catalyst-repelling material include, but are not limited to,SiO₂, and anodic alumina. As noted throughout, the temperature andprecursor gases can be varied throughout the growth process, suitablystarting at a higher temperature initially, and then using a lowertemperature for continued nanowire growth. Examples of precursor gasmixtures for use in the methods of the present invention include, butare not limited to, SiH₄, SiCl₄ and SiH₂Cl₂. The present invention alsoprovides nanowires produced by the methods of the present invention andelectronic devices comprising such nanowires.

Additional methods for producing nanowires on patterned substrates arealso provided. Suitably, a catalyst-repelling material is applied on asubstrate material to at least partially cover the substrate material.Then, one or more nucleating particles are applied on the substratematerial. The nucleating particles are then heated (e.g., to atemperature of above about 400° C.) and then contacted with a precursorgas mixture (e.g., at a pressure of above about 0.5 torr) to create analloy droplet, whereby nanowires are grown at the site of the alloydroplet. Suitably, the catalyst-repelling material (e.g., SiO₂ or anodicalumina) comprises at least one void that does not cover the substratematerial (e.g., a silicon or other crystallographic substrate). Inexemplary embodiments, nucleating particles are deposited on thecatalyst-repelling material (e.g., in the form of a film or colloid andthen heated), wherein the nucleating particles deposit on the substratematerial at the site of the at least one void.

In exemplary embodiments, the methods of the present invention compriseheating to a temperature of about 450° C. to about 700° C., at apressure of about 5 to about 200 torr, suitably about 45 torr. Exemplarynucleating particles include those disclosed throughout, such as metalscomprising Au, Al, Pt, Fe, Ti, Ga, Ni, Sn or In. Exemplary precursor gasmixtures include those disclosed throughout, such as gas mixturescomprising SiH₄, Si₂H₆, SiCl₄ or SiH₂Cl₂.

The present invention also provides additional methods for producingnanowires. For example, one or more nucleating particles (e.g., metalliccolloids or films) are be applied on a substrate material. Then, thenucleating particles are heated to a temperature of greater than about400° C. (suitably, between about 450° C. to about 700° C.), andcontacted with a precursor gas mixture at a pressure greater than about0.5 torr (suitably between about 5 to about 200 torr, more suitablyabout 45 torr) to create a liquid alloy droplet, whereby nanowires aregrown at the site of the alloy droplet.

Exemplary nucleating particles include those disclosed throughout, suchas metals comprising Au, Al, Pt, Fe, Ti, Ga, Ni, Sn or In. Exemplaryprecursor gas mixtures include those disclosed throughout, such as gasmixtures comprising SiH₄, Si₂H₆, SiCl₄ or SiH₂Cl₂.

Further embodiments, features, and advantages of the invention, as wellas the structure and operation of the various embodiments of theinvention are described in detail below with reference to accompanyingdrawings.

BRIEF DESCRIPTION OF THE FIGURES

The invention is described with reference to the accompanying drawings.In the drawings, like reference numbers indicate identical orfunctionally similar elements. The drawing in which an element firstappears is indicated by the left-most digit in the correspondingreference number.

FIG. 1A is a diagram of a single crystal semiconductor nanowire.

FIG. 1B is a diagram of a nanowire doped according to a core-shellstructure.

FIG. 2 is a flowchart of a method for preparing nanowires using acombination of Si precursors, according to an embodiment of theinvention.

FIG. 3 is a flowchart of a method for preparing nanowires using acombination of Si precursors, according to an embodiment of theinvention.

FIG. 4 is a flowchart of a method for nanowire synthesis, according toan embodiment of the invention.

FIG. 4A is a diagram of a nanowire synthesis vessel, according to anembodiment of the invention.

FIG. 5 is a flowchart of a method for doping nanowires, according to anembodiment of the invention.

FIG. 6 is a flowchart of a method for reducing surface states fromdangling bonds on a nanowire structure, according to an embodiment ofthe invention.

FIG. 7 is a diagram of a nanowire coated with a sacrificial layer,according to an embodiment of the invention.

FIG. 8 is a diagram showing nanowire processing in accordance with oneembodiment of the present invention.

FIG. 9 is a diagram showing nanowire processing following transfer inaccordance with one embodiment of the present invention.

FIG. 10 is a flowchart of a method of harvesting a nanowire using asacrificial portion, according to an embodiment of the invention.

FIG. 11 is a diagram of a nanowire with a sacrificial portion, accordingto an embodiment of the invention.

FIG. 12 is a flowchart of a method of harvesting a nanowire bymonitoring of a PN junction, according to an embodiment of theinvention.

FIG. 13 is a flowchart of a method of harvesting a nanowire using asacrificial metal layer on a nanowire growth substrate, according to anembodiment of the invention.

FIG. 14 is a diagram of a nanowire grown on a sacrificial metal layer ona nanowire growth substrate, according to an embodiment of theinvention.

FIG. 15 is a flowchart of a method of harvesting an Si nanowire whenusing a non-Si substrate, according to an embodiment of the invention.

FIG. 16 is a flowchart of a method of harvesting a nanowire with oneorientation when a nanowire growth substrate with a differentorientation is used, according to an embodiment of the invention.

FIG. 17 is a diagram of a nanowire with one orientation growing on ananowire growth substrate with a different orientation, according to anembodiment of the invention.

FIG. 18 is a flowchart of a method for transferring nanowires from afirst substrate to a second substrate, according to an embodiment of theinvention.

FIG. 19 is a flowchart of a method for transferring nanowires from afirst substrate with a patterned coating to a second substrate,according to an embodiment of the invention.

FIG. 20A is a diagram of a first substrate with nanowires and a transfersubstrate, according to an embodiment of the invention.

FIG. 20B is a diagram of a device substrate and a transfer substratetransferring nanowires, according to an embodiment of the invention.

FIG. 21 is a representation of probe nanowire transfer scheme inaccordance with one embodiment of the present invention.

FIG. 22 is a representation of global nanowire transfer in accordancewith one embodiment of the present invention.

FIG. 23A is a transmission electron micrograph that shows a substratewith e-field oriented nanowires prior to transfer, according to anembodiment of the invention.

FIG. 23B is a transmission electron micrograph that shows nanowiresremaining on substrate following transfer, according to an embodiment ofthe invention.

FIG. 23C is a transmission electron micrograph that shows nanowires ontransfer substrate following transfer, according to an embodiment of theinvention.

FIG. 24 is a diagram showing oriented nanowire growth using a patternedsubstrate in accordance with one embodiment of the present invention.

FIG. 25 is a flowchart of a method for preparing nanowires on apatterned substrate accordance with one embodiment of the presentinvention.

FIG. 26 is a flowchart of a method for preparing nanowires in accordancewith one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

It should be appreciated that the particular implementations shown anddescribed herein are examples of the invention and are not intended tootherwise limit the scope of the present invention in any way. Indeed,for the sake of brevity, conventional electronics, manufacturing,semiconductor devices, and nanowire (NW), nanorod, nanotube, andnanoribbon technologies and other functional aspects of the systems (andcomponents of the individual operating components of the systems) maynot be described in detail herein. Furthermore, for purposes of brevity,the invention is frequently described herein as pertaining to nanowires.

It should be appreciated that although nanowires are frequently referredto, the techniques described herein are also applicable to othernanostructures, such as nanorods, nanotubes, nanotetrapods, nanoribbonsand/or combinations thereof. It should further be appreciated that themanufacturing techniques described herein could be used to create anysemiconductor device type, and other electronic component types.Further, the techniques would be suitable for application in electricalsystems, optical systems, consumer electronics, industrial electronics,wireless systems, space applications, or any other application.

As used herein, an “aspect ratio” is the length of a first axis of ananostructure divided by the average of the lengths of the second andthird axes of the nanostructure, where the second and third axes are thetwo axes whose lengths are most nearly equal to each other. For example,the aspect ratio for a perfect rod would be the length of its long axisdivided by the diameter of a cross-section perpendicular to (normal to)the long axis.

The term “heterostructure” when used with reference to nanostructuresrefers to nanostructures characterized by at least two different and/ordistinguishable material types. Typically, one region of thenanostructure comprises a first material type, while a second region ofthe nanostructure comprises a second material type. In certainembodiments, the nanostructure comprises a core of a first material andat least one shell of a second (or third etc.) material, where thedifferent material types are distributed radially about the long axis ofa nanowire, a long axis of an arm of a branched nanocrystal, or thecenter of a nanocrystal, for example. A shell need not completely coverthe adjacent materials to be considered a shell or for the nanostructureto be considered a heterostructure. For example, a nanocrystalcharacterized by a core of one material covered with small islands of asecond material is a heterostructure. In other embodiments, thedifferent material types are distributed at different locations withinthe nanostructure. For example, material types can be distributed alongthe major (long) axis of a nanowire or along a long axis of arm of abranched nanocrystal. Different regions within a heterostructure cancomprise entirely different materials, or the different regions cancomprise a base material.

As used herein, a “nanostructure” is a structure having at least oneregion or characteristic dimension with a dimension of less than about500 nm, e.g., less than about 200 nm, less than about 100 nm, less thanabout 50 nm, or even less than about 20 nm. Typically, the region orcharacteristic dimension will be along the smallest axis of thestructure. Examples of such structures include nanowires, nanorods,nanotubes, branched nanocrystals, nanotetrapods, tripods, bipods,nanocrystals, nanodots, quantum dots, nanoparticles, branched tetrapods(e.g., inorganic dendrimers), and the like. Nanostructures can besubstantially homogeneous in material properties, or in certainembodiments can be heterogeneous (e.g., heterostructures).Nanostructures can be, for example, substantially crystalline,substantially monocrystalline, polycrystalline, amorphous, or acombination thereof. In one aspect, each of the three dimensions of thenanostructure has a dimension of less than about 500 nm, for example,less than about 200 nm, less than about 100 nm, less than about 50 nm,or even less than about 20 nm.

As used herein, the term “nanowire” generally refers to any elongatedconductive or semiconductive material (or other material describedherein) that includes at least one cross sectional dimension that isless than 500 nm, and preferably, less than 100 nm, and has an aspectratio (length:width) of greater than 10, preferably greater than 50, andmore preferably, greater than 100.

The nanowires of this invention can be substantially homogeneous inmaterial properties, or in certain embodiments can be heterogeneous(e.g. nanowire heterostructures). The nanowires can be fabricated fromessentially any convenient material or materials, and can be, e.g.,substantially crystalline, substantially monocrystalline,polycrystalline, or amorphous. Nanowires can have a variable diameter orcan have a substantially uniform diameter, that is, a diameter thatshows a variance less than about 20% (e.g., less than about 10%, lessthan about 5%, or less than about 1%) over the region of greatestvariability and over a linear dimension of at least 5 nm (e.g., at least10 nm, at least 20 nm, or at least 50 nm). Typically the diameter isevaluated away from the ends of the nanowire (e.g. over the central 20%,40%, 50%, or 80% of the nanowire). A nanowire can be straight or can bee.g. curved or bent, over the entire length of its long axis or aportion thereof. In certain embodiments, a nanowire or a portion thereofcan exhibit two- or three-dimensional quantum confinement. Nanowiresaccording to this invention can expressly exclude carbon nanotubes, and,in certain embodiments, exclude “whiskers” or “nanowhiskers”,particularly whiskers having a diameter greater than 100 nm, or greaterthan about 200 nm.

Examples of such nanowires include semiconductor nanowires as describedin Published International Patent Application Nos. WO 02/17362, WO02/48701, and WO 01/03208, carbon nanotubes, and other elongatedconductive or semiconductive structures of like dimensions, which areincorporated herein by reference.

As used herein, the term “nanorod” generally refers to any elongatedconductive or semiconductive material (or other material describedherein) similar to a nanowire, but having an aspect ratio (length:width)less than that of a nanowire. Note that two or more nanorods can becoupled together along their longitudinal axis so that the couplednanorods span all the way between electrodes. Alternatively, two or morenanorods can be substantially aligned along their longitudinal axis, butnot coupled together, such that a small gap exists between the ends ofthe two or more nanorods. In this case, electrons can flow from onenanorod to another by hopping from one nanorod to another to traversethe small gap. The two or more nanorods can be substantially aligned,such that they form a path by which electrons can travel betweenelectrodes.

A wide range of types of materials for nanowires, nanorods, nanotubesand nanoribbons can be used, including semiconductor material selectedfrom, e.g., Si, Ge, Sn, Se, Te, B, C (including diamond), P, B—C,B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN/BP/BAs, AlN, AlP,AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, ZnO, ZnS, ZnSe,ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, BeS, BeSe, BeTe, MgS, MgSe, GeS,GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr, CuI,AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂, CuGeP₃,CuSi₂P₃, (Cu,Ag)(Al,Ga,In,Tl,Fe)(S,Se,Te)₂, Si₃N₄, Ge₃N₄, Al₂O₃,(Al,Ga,In)₂ (S,Se,Te)₃, Al₂CO, and an appropriate combination of two ormore such semiconductors.

The nanowires can also be formed from other materials such as metalssuch as gold, nickel, palladium, iridium, cobalt, chromium, aluminum,titanium, tin and the like, metal alloys, polymers, conductive polymers,ceramics, and/or combinations thereof. Other now known or laterdeveloped conducting or semiconductor materials can be employed.

In certain aspects, the semiconductor may comprise a dopant from a groupconsisting of: a p-type dopant from Group III of the periodic table; ann-type dopant from Group V of the periodic table; a p-type dopantselected from a group consisting of: B, Al and In; an n-type dopantselected from a group consisting of: P, As and Sb; a p-type dopant fromGroup II of the periodic table; a p-type dopant selected from a groupconsisting of: Mg, Zn, Cd and Hg; a p-type dopant from Group IV of theperiodic table; a p-type dopant selected from a group consisting of: Cand Si; or an n-type dopant selected from a group consisting of: Si, Ge,Sn, S, Se and Te. Other now known or later developed dopant materialscan be employed.

Additionally, the nanowires or nanoribbons can include carbon nanotubes,or nanotubes formed of conductive or semiconductive organic polymermaterials, (e.g., pentacene, and transition metal oxides).

Hence, although the term “nanowire” is referred to throughout thedescription herein for illustrative purposes, it is intended that thedescription herein also encompass the use of nanotubes (e.g.,nanowire-like structures having a hollow tube formed axiallytherethrough). Nanotubes can be formed in combinations/thin films ofnanotubes as is described herein for nanowires, alone or in combinationwith nanowires, to provide the properties and advantages describedherein.

It should be understood that the spatial descriptions (e.g., “above”,“below”, “up”, “down”, “top”, “bottom,” etc.) made herein are forpurposes of illustration only, and that devices of the present inventioncan be spatially arranged in any orientation or manner. In addition,there may also be intervening layers or materials present in suchdevices to facilitate processing.

Types of Nanowires and their Synthesis

FIG. 1A illustrates a single crystal semiconductor nanowire core(hereafter “nanowire”) 100. FIG. 1A shows a nanowire 100 that is auniformly doped single crystal nanowire. Such single crystal nanowirescan be doped into either p- or n-type semiconductors in a fairlycontrolled way. Doped nanowires such as nanowire 100 exhibit improvedelectronic properties. For instance, such nanowires can be doped to havecarrier mobility levels comparable to bulk single crystal materials.

FIG. 11B shows a nanowire 110 doped according to a core-shell structure.As shown in FIG. 11B, nanowire 110 has a doped surface layer 112, whichcan have varying thickness levels, including being only a molecularmonolayer on the surface of nanowire 110.

The valence band of the insulating shell can be lower than the valenceband of the core for p-type doped wires, or the conduction band of theshell can be higher than the core for n-type doped wires. Generally, thecore nanostructure can be made from any metallic or semiconductormaterial, and the shell can be made from the same or a differentmaterial. For example, the first core material can comprise a firstsemiconductor selected from the group consisting of: a Group II-VIsemiconductor, a Group III-V semiconductor, a Group IV semiconductor,and an alloy thereof. Similarly, the second material of the shell cancomprise a second semiconductor, the same as or different from the firstsemiconductor, e.g., selected from the group consisting of: a GroupII-VI semiconductor, a Group III-V semiconductor, a Group IVsemiconductor, and an alloy thereof. Example semiconductors include, butare not limited to, CdSe, CdTe, InP, InAs, CdS, ZnS, ZnSe, ZnTe, HgTe,GaN, GaP, GaAs, GaSb, InSb, Si, Ge, AlAs, AlSb, PbSe, PbS, and PbTe. Asnoted above, metallic materials such as gold, chromium, tin, nickel,aluminum etc. and alloys thereof can be used as the core material, andthe metallic core can be overcoated with an appropriate shell materialsuch as silicon dioxide or other insulating materials

Nanostructures can be fabricated and their size can be controlled by anyof a number of convenient methods that can be adapted to differentmaterials. For example, synthesis of nanocrystals of various compositionis described in, e.g., Peng et al. (2000) “Shape Control of CdSeNanocrystals” Nature 404, 59-61; Puntes et al. (2001) “Colloidalnanocrystal shape and size control: The case of cobalt” Science 291,2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos et al. (Oct. 23, 2001)entitled “Process for forming shaped group III-V semiconductornanocrystals, and product formed using process”; U.S. Pat. No. 6,225,198to Alivisatos et al. (May 1, 2001) entitled “Process for forming shapedgroup II-VI semiconductor nanocrystals, and product formed usingprocess”; U.S. Pat. No. 5,505,928 to Alivisatos et al. (Apr. 9, 1996)entitled “Preparation of III-V semiconductor nanocrystals”; U.S. Pat.No. 5,751,018 to Alivisatos et al. (May 12, 1998) entitled“Semiconductor nanocrystals covalently bound to solid inorganic surfacesusing self-assembled monolayers”; U.S. Pat. No. 6,048,616 to Gallagheret al. (Apr. 11, 2000) entitled “Encapsulated quantum sized dopedsemiconductor particles and method of manufacturing same”; and U.S. Pat.No. 5,990,479 to Weiss et al. (Nov. 23, 1999) entitled “Organoluminescent semiconductor nanocrystal probes for biological applicationsand process for making and using such probes.”

Growth of nanowires having various aspect ratios, including nanowireswith controlled diameters, is described in, e.g., Gudiksen et al (2000)“Diameter-selective synthesis of semiconductor nanowires” J. Am. Chem.Soc. 122, 8801-8802; Cui et al. (2001) “Diameter-controlled synthesis ofsingle-crystal silicon nanowires” Appl. Phys. Lett. 78, 2214-2216;Gudiksen et al. (2001) “Synthetic control of the diameter and length ofsingle crystal semiconductor nanowires” J. Phys. Chem. B 105, 4062-4064;Morales et al. (1998) “A laser ablation method for the synthesis ofcrystalline semiconductor nanowires” Science 279, 208-211; Duan et al.(2000) “General synthesis of compound semiconductor nanowires” Adv.Mater. 12, 298-302; Cui et al. (2000) “Doping and electrical transportin silicon nanowires” J. Phys. Chem. B 104, 5213-5216; Peng et al.(2000) “Shape control of CdSe nanocrystals” Nature 404, 59-61; Puntes etal. (2001) “Colloidal nanocrystal shape and size control: The case ofcobalt” Science 291, 2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos etal. (Oct. 23, 2001) entitled “Process for forming shaped group III-Vsemiconductor nanocrystals, and product formed using process”; U.S. Pat.No. 6,225,198 to Alivisatos et al. (May 1, 2001) entitled “Process forforming shaped group II-VI semiconductor nanocrystals, and productformed using process”; U.S. Pat. No. 6,036,774 to Lieber et al. (Mar.14, 2000) entitled “Method of producing metal oxide nanorods”; U.S. Pat.No. 5,897,945 to Lieber et al. (Apr. 27, 1999) entitled “Metal oxidenanorods”; U.S. Pat. No. 5,997,832 to Lieber et al. (Dec. 7, 1999)“Preparation of carbide nanorods”; Urbau et al. (2002) “Synthesis ofsingle-crystalline perovskite nanowires composed of barium titanate andstrontium titanate” J. Am. Chem. Soc., 124, 1186; and Yun et al. (2002)“Ferroelectric Properties of Individual Barium Titanate NanowiresInvestigated by Scanned Probe Microscopy” Nanoletters 2, 447.

Growth of branched nanowires (e.g., nanotetrapods, tripods, bipods, andbranched tetrapods) is described in, e.g., Jun et al. (2001) “Controlledsynthesis of multi-armed CdS nanorod architectures using monosurfactantsystem” J. Am. Chem. Soc. 123, 5150-5151; and Manna et al. (2000)“Synthesis of Soluble and Processable Rod-, Arrow-, Teardrop-, andTetrapod-Shaped CdSe Nanocrystals” J. Am. Chem. Soc. 122, 12700-12706.

Synthesis of nanoparticles is described in, e.g., U.S. Pat. No.5,690,807 to Clark Jr. et al. (Nov. 25, 1997) entitled “Method forproducing semiconductor particles”; U.S. Pat. No. 6,136,156 to El-Shall,et al. (Oct. 24, 2000) entitled “Nanoparticles of silicon oxide alloys”;U.S. Pat. No. 6,413,489 to Ying et al. (Jul. 2, 2002) entitled“Synthesis of nanometer-sized particles by reverse micelle mediatedtechniques”; and Liu et al. (2001) “Sol-Gel Synthesis of Free-StandingFerroelectric Lead Zirconate Titanate Nanoparticles” J. Am. Chem. Soc.123, 4344. Synthesis of nanoparticles is also described in the abovecitations for growth of nanocrystals, nanowires, and branched nanowires,where the resulting nanostructures have an aspect ratio less than about1.5.

Synthesis of core-shell nanostructure heterostructures, namelynanocrystal and nanowire (e.g., nanorod) core-shell heterostructures,are described in, e.g., Peng et al. (1997) “Epitaxial growth of highlyluminescent CdSe/CdS core/shell nanocrystals with photostability andelectronic accessibility” J. Am. Chem. Soc. 119, 7019-7029; Dabbousi etal. (1997) “(CdSe)ZnS core-shell quantum dots: Synthesis andcharacterization of a size series of highly luminescent nanocrysallites”J. Phys. Chem. B 101, 9463-9475; Manna et al. (2002) “Epitaxial growthand photochemical annealing of graded CdS/ZnS shells on colloidal CdSenanorods” J. Am. Chem. Soc. 124, 7136-7145; and Cao et al. (2000)“Growth and properties of semiconductor core/shell nanocrystals withInAs cores” J. Am. Chem. Soc. 122, 9692-9702. Similar approaches can beapplied to growth of other core-shell nanostructures.

Growth of nanowire heterostructures in which the different materials aredistributed at different locations along the long axis of the nanowireis described in, e.g., Gudiksen et al. (2002) “Growth of nanowiresuperlattice structures for nanoscale photonics and electronics” Nature415, 617-620; Bjork et al. (2002) “One-dimensional steeplechase forelectrons realized” Nano Letters 2, 86-90; Wu et al. (2002)“Block-by-block growth of single-crystalline Si/SiGe superlatticenanowires” Nano Letters 2, 83-86; and U.S. patent application 60/370,095(Apr. 2, 2002) to Empedocles entitled “Nanowire heterostructures forencoding information.” Similar approaches can be applied to growth ofother heterostructures.

Epitaxial-Oriented Nanowire Growth Using a Combination of SiliconPrecursors

FIG. 2 is a flowchart of method 200 for preparing nanowires using acombination of Si precursors, according to an embodiment of theinvention. Method 200 begins in step 202. In step 202, one or morenucleating particles, suitably metal catalysts, are deposited on asubstrate material to create a nucleation site for nanowire growth. Asshown in step 204, heating of the nucleating particles to a firsttemperature and contacting the nucleating particles with a firstprecursor gas mixture, creates a liquid alloy droplet and initiatesnanowire growth, which is indicated by label 206. In step 208 heatingthe alloy droplet to a second temperature and contacting the alloydroplet with a second precursor gas mixture, allows nanowires to grow atthe site of the alloy droplet, which is indicated by label 210, untilthey reach the desired size and orientation, as shown in step 214.

In suitable embodiments, the substrate material on which the nanowiresare grown is a crystallographic substrate. The term “crystallographicsubstrate” includes any substrate material which comprises atomssituated in a repeating or periodic array over large atomic distances,typically on the order of 10 or more angstroms (Å). Suchcrystallographic substrates may be polycrystalline or may comprisesingle crystals. Suitably, the crystallographic substrate utilized inthe processes of the present invention is silicon (Si). Other suitablecrystallographic materials included, but are not limited to, germanium(Ge), gallium arsenide (GaAs), gallium nitride (GaN), sapphire, quartz,and silicon germanium (SiGe). In other embodiments of the presentinvention, the substrate material may comprise an amorphous material.Suitable amorphous substrate materials which may be used in the practiceof the present invention include, but are not limited to SiO₂, Si₃N₄ andalumina. In additional embodiments, substrate materials are composed ofmultiple layers, such as a silicon on insulator (SOI) surface.

As outlined in FIG. 2, in certain embodiments, the methods of thepresent invention comprise first depositing nucleating particles on asubstrate material. Nucleating particles that may be used in thepractice of the present invention include metal catalysts and can be anymetal that reacts with both the first precursor gas mixture and thesecond precursor gas mixture (i.e., decomposed forms of the first andsecond precursor gas mixtures) to form a eutectic phase. Such a mixturehas a minimum melting point at which all components are in solution.Upon addition and decomposition of precursor gas molecules (e.g.,silicon) a saturation point on the eutectic phase diagram is reachedsuch that semiconductor particles (e.g., Si) begin to precipitate out ofthe metal solution, thereby creating a growing nanowire. Continuousaddition of precursor gas, as it decomposes, will continue to saturatethe eutectic, thereby generating additional material for nanowiregrowth.

In suitable embodiments, the nucleating particles will be metalcatalysts and can comprise any of the transition metals from thePeriodic Table, including, but not limited to, copper, silver, aluminumgold, nickel, palladium, platinum, cobalt, rhodium, iridium, iron,ruthenium, tin, osmium, manganese, chromium, molybdenum, tungsten,vanadium, niobium, tantalum, titanium, zirconium and gallium, includingmixtures of one or more of these metals. In preferred embodiments of thepresent invention, the metal catalyst can comprise a gold (Au) colloid(i.e., a Au nanoparticle) or Au film. In certain such embodiments, 60nanometer (nm) diameter gold colloids can be used. The target is toachieve a uniform deposition of gold nanoparticles with density between2-4 particles per square micrometer (μm). A key is minimized goldparticle cluster formation. The clusters can result in undesired largerdiameter nanowire growth. Spin coating and self assembly methods can beexplored for the deposition (see e.g., U.S. patent application Ser. No.10/674,060, filed Sep. 30, 2003, which incorporated by reference hereinin its entirety).

Spin coating is a fairly straightforward process. A deposition densitycan be controlled through variation of the gold particle concentrationin the precursor colloids, manipulation of surface chemistry of thesilicon wafer, and changing the spin speed. A drawback of spin coatingcan be low utilization efficiency of gold colloid solution. A recyclingprocess in the production stage can be used if warranted.

Self assembly involves some use of well established chemistry. Thesurface of 4 inch silicon oxide coated wafer is functionalized witheither (3-aminopropyl)-trimethoxysilane (APTMS) or(3-mercaptopropyl)-trimethoxysilane (MPTMS), then contacted with 60 nmgold colloid solution. The gold particles are assembled on the surface.The difference between two different chemistries are compared, and thepossibility of controlling the density of gold particles by control ofthe contact time and gold particle concentration in the contact solutioncan be used.

The nucleating particles used to practice the present invention can alsobe formed on a substrate surface by heating a gold film coating layer onthe surface.

In one embodiment, the present invention comprises heating the firstprecursor gas mixture to a temperature at which 1) the gas dissociatesinto its free component atoms, and 2) the nucleating particles (e.g.metal catalyst) melts to a liquid. The free gas molecules can thendiffuse into the metal catalyst to form a liquid alloy droplet. Thisprocess is commonly known to those of ordinary skill in the art aschemical vapor deposition (CVD).

In suitable embodiments of the present invention, the first precursorgas may comprise a gas which includes at least one atomic species thatpromotes the growth of nanowires (e.g., Si) as well as an atomic speciesthat aids in orienting the nanowires during their growth (e.g., Clatoms). For example, the first precursor gas may be selected from, butnot limited to, Si₂H₆, SiH₄, SiCl₄ and SiH₂Cl₂ gas, preferably SiCl₄ orSiH₂Cl₂.

Heating these Si precursor gases above the temperature at which thethermal energy is sufficient to break the bond energies between thegaseous molecules generates free Si atoms. (e.g., Si—H bond: 93kcal/mole, Si—Cl bond: 110 kcal/mole, Si—Si bond; 77 kcal/mole, see M.T. Swihart and R. W. Carr, J. Phys Chem A 102:1542-1549 (1998).)Provided that this temperature is also high enough to liquefy the metalcatalyst, the free Si atoms will diffuse into the metal and generate aeutectic phase. Dissociation temperatures for SiH₄ and Si₂H₆, and SiCl₄and SiH₂Cl₂ gases are between about 300° C. to 500° C. (for Si₂H₆ andSiH₄), and above about 800° C. (for SiCl₄ and SiH₂Cl₂) respectively. Inthe instances of SiCl₄ or SiH₂Cl₂, Cl atoms are also generated.Decomposition of SiCl₄ or SiH₂Cl₂ into Si and Cl in the presence of acarrier gas (e.g., H₂, H₂Ar) forms HCl.

As discussed in more detail below, this creates a competition betweenetching with HCl and growth from the Si vapor. Chlorine aids in removalof interfacial oxide on Si substrates leading to oriented NW growth.However, because the decomposition of SiCl₄ or SiH₂Cl₂ into Si and Cloccurs at a relatively high temperature (e.g., above about 800° C.),metal diffusion of the metal catalysts into the growing nanowires ismore likely to occur if the temperature is maintained above about 800°C. for an extended time period.

Furthermore, at higher temperatures, the loss of metal catalyst is morelikely to occur due to increased melting of the catalyst leaving lesscatalyst (or none) available to promote growth of the nanowires, thuslimiting the length to which the nanowires can be grown. Accordingly,following the initiation of nanowire growth and orientation with SiCl₄or SiH₂Cl₂, it is preferable to introduce another precursor gas mixture(including, e.g., Si₂H₆ or SiH₄) to contact the metal catalyst, whichgas mixture includes a precursor gas which decomposes into Si atoms atlower temperatures than the first precursor gas (but at a high enoughtemperature to form a eutectic phase with the metal catalyst).

In all embodiments of the present invention, the precursor gas mixturesused during any of the nanowire growth processes may further compriseone or more doping gases. Examples of suitable doping gases that may beused in the practice of the present invention include, but are notlimited to, B₂H₆, trimethyl boron (TMB), POCl₃ and PH₃.

In one embodiment of the present invention, the first precursor gas cancomprise SiCl₄ and suitably a carrier gas, such as He, Ar, or otherinert gas. Heating this gas mixture to a sufficiently high temperature,e.g., above about 800° C., generates free Si and Cl atoms. In suitablesuch embodiments, the first precursor gas may comprise one or moredopant gases selected from those described throughout the application.The first precursor gas mixture is passed over the nucleating particles,suitably metal-catalyst particles (e.g., gold nanoparticles) depositedon the substrate material at a total pressure between about 20 to about50 Torr, while the nucleating particles are heated up to a temperatureof about 800° C. In other embodiments of the present invention, the gaspressure may be increased or decreased, thereby requiring a modificationin the temperature required to dissociate the precursor gas mixture.

For example, SiCl₄ when heated to the appropriate temperature willdecompose into free Si and Cl atoms. When B₂H₆ is present in theprecursor gas mixture, B atoms will also be generated. Si and B willdiffuse into the metal catalyst and generate a liquid alloy droplet.This eutectic phase of metal catalyst and precursor gases will continueto exist as precursor gas is solvated in the metal catalyst. Once anover-saturation is reached, Si/B atoms will precipitate out and initiatenanowire growth. In order to continue nanowire growth, a continuoussupply of Si precursor gas and doping gas are required. However,maintaining a temperature above about 800° C. for extended periods canlead to the metal catalyst diffusing into the growing nanowire creatingtrap states and reducing the diameter and length of the overall nanowirethat can be obtained.

In certain embodiments of the present invention, once nanowire growth isinitiated (e.g., using SiCl₄ or SiH₂Cl₂), a second precursor gas can besubstituted (e.g., SiH₄) and nanowire growth continued. The point atwhich the second precursor gas is introduced into the system can bedetermined experimentally by the skilled artisan. In other embodimentsof the present invention, nanowire growth can be initiated using SiH₄and then followed with SiCl₄ or SiH₂Cl₂ as a second precursor gasthereby allowing for Cl etching. In certain such embodiments, the firsttemperature at which the first precursor gas mixture is added toinitiate nanowire growth and the second temperature at which nanowiregrowth is continued using a second precursor gas mixture can be thesame, so long as the temperature used is high enough to allow fordissociation of the gases and diffusion of Si and dopant into the liquidmetal catalyst. In other embodiments, the first temperature used toinitiate nanowire growth will be higher than the second temperature usedto continue nanowire growth. These temperatures can differ by any amount(e.g., about a few degrees C. to 10's of degrees C. to 100's of degreesC.), so long as the temperatures used are high enough to allow fordissociation of the gases and diffusion of Si and dopant into the liquidmetal catalyst.

In certain embodiments of the present invention, once nanowire growth isinitiated, suitably using a temperature of about 800° C. and SiCl₄ orSiH₂Cl₂ as a first precursor gas, a second precursor gas can besubstituted and the temperature changed to continue nanowire growth. Thepoint at which the second precursor gas is introduced into the systemcan be determined experimentally by the skilled artisan. In certain suchembodiments, the first temperature at which nanowire growth is initiatedwill be higher than the second temperature, where nanowire growthcontinues. In suitable embodiments, the second precursor gas willcomprise SiH₄ and growth will continue at a temperature of about 300° C.to about 500° C. In certain embodiments, the first temperature will beat least about 50° C. above the second temperature. Continuing to growthe nanowire at a lower temperature reduces the possibility that themetal catalyst can diffuse into the growing nanowire.

The present invention encompasses the use of any number of precursorgases in the process of growing nanowires. For example, nanowire growthcan be initiated with a first precursor gas comprising SiCl₄, suitablyat a temperature of about 800° C., and then continued with a secondprecursor gas comprising SiH₂Cl₂, suitably at a temperature of about750° C. In other embodiments, SiH₂Cl₂ can be used as the first precursorgas. The wires then can be contacted with a third precursor gas,comprising, for example SiH₄ or Si₂H₆, suitably at a temperature ofabout 300-500° C. As shown in FIG. 2 in step 212, in other embodimentsof the invention, any number of precursor gases can be introduced to thenanowires during the initiation and growth processes, until thenanowires reach the desired size and orientation, which is indicated instep 214. Provided that as long as the temperature at which theprecursor gases contact the metal catalyst is above the dissociationtemperature of the gas mixture and above the temperature required toliquefy the metal catalyst, the wires will continue to grow, as Si (orother suitable semiconductor materials as discussed throughout) and thedopant will continue to precipitate out of the metal catalyst allowdroplet.

In certain embodiments, nanowire growth can be initiated using SiH₄ orSi₂H₆ and then continued using SiCl₄ or SiH₂Cl₂ to allow for Cl etchingof Si growing in undesired locations and orientations on the substratesurface. The precursor gas mixture may then be switched back to SiH₄ orSi₂H₆ if desired. In certain such embodiments, the temperature at whichthe three gases can be kept the same, or can be modified as needed, solong as free Si atoms are allowed to mix with the liquefied metalcatalyst. In other embodiments of the present invention, free Si, Cl orH atoms may be supplied to the growing nanowires to aid in growth (e.g.,Si), etching (e.g., Cl) or gas dissociation (e.g., H) as needed.

As discussed throughout, the precursor gas mixtures used in theprocesses of the present invention may further comprise a dopant. Insuitable embodiments of the present invention, wires can be grown usingthe same doping gas in each of the precursor gas mixtures. In suchembodiments, the entire resulting wire will be either p-type or n-type,depending on the choice of dopant. In other embodiments of the presentinvention, different doping gases can be introduced throughout theprocess as components of the different precursor gases. For example,wire growth can be initiated using a precursor gas comprising a n-typedopant (e.g., P, As or Sb) and then continued using a precursor gasusing a p-type dopant (e.g., B, Al or In). In other embodiments, ap-type doing gas will be used during initiation and then an n-typedoping gas during growth.

In other embodiments, the type of doping gas can be switched multipletimes throughout the growth process as the precursor gases are switched.The resulting nanowires therefore can comprise several different dopantportions throughout their length. For example, a nanowire produced viathe present invention may comprise an n-type base, a p-type middlesection, and an n-type top, or any suitable combination as envisioned bythe ordinarily skilled artisan. Such embodiments of the presentinvention would allow for an n-type wire to be grown on a p-typesubstrate, and vice versa.

Continuously supplying the second precursor gas mixture (and third,fourth, fifth, etc.) will allow the nanowire to continue growing untiltermination by desire or death caused by local condition change. Thequality of the nanowires is dependent on the quality of goldnanoparticles, control of gold nanoparticle distribution on thesubstrate and growth condition including temperature, ratio of dopant toprecursor gas, partial pressure of the precursor gas, and resident timeof precursor gases in the reactor. In suitable embodiments of thepresent invention, the processes of the present invention can beaccomplished using a computer controlled 8″ semiconductor furnace.

In suitable embodiments, the various precursor gas mixtures that areintroduced in any of the processes of the present invention may beintroduced via Plasma Enhanced Sputter Deposition (or Plasma EnhancedChemical Vapor Deposition (PECVD)) and the processes performed at lowertemperatures. (See Hofmann et al., “Gold Catalyzed Growth of SiliconNanowires by Plasma Enhanced Chemical Vapor Deposition,” J. Appl. Phys.94:6005-6012 (2003).) Decomposition of SiCl₄ or SiH₂Cl₂ into Si and Clin the presence of a carrier gas (e.g., H₂, H₂Ar) forms HCl. Thiscreates a competition between etching with HCl and growth from the Sivapor. Chlorine aids in removal of interfacial oxide on Si substratesleading to the oriented NW growth. Loss of metal catalyst (e.g., Au) canoccur either by etching or thermal evaporation of AuCl that can form.Use of PECVD to grow NWs at temperatures below about 800° C., with theaddition of chlorine gas from SiCl₄ or SiH₂Cl₂, and/or the addition ofchlorine gas from a source separate from the source of SiCl₄ or SiH₂Cl₂,the reactive species of Si and Cl can be independently controlled in theplasma to enhance or suppress etching as needed to promote nanowiregrowth. Sputter deposition can be accomplished via any method known tothe ordinarily skilled artisan, for example, diode, radio frequency anddirect current deposition.

The diameter distribution of silicon nanowires of these certainembodiments of the present invention is determined by that of thenucleating particles, e.g., metal (suitably gold) nanoparticles.Commercially available 60 nanometer gold colloids can have a diameterdistribution of ±10%. The same distribution can be attained in thenanowires. Gold nanoparticles can be split into smaller ones resultingin smaller diameter nanowires, depending on the growth condition. Growthconditions can be optimized to minimize this event. Given a growthcondition, the length of nanowires can be controlled by varying durationof the growth. Crystallinity of silicon nanowires and dopantconcentration are also growth condition dependent. They can be optimizedand controlled together with other important nanowire characteristics.

The nanowires produced according to any of the processes of the presentinvention will suitably grow out of the plane of the substrate material.Such growth includes nanowires that project out of the plane of thesubstrate material at any angle with respect to the substrate. Forexample, nanowires can grow at an angle of about 1° to about 90°, andany angle in between these values, relative to the plane of thesubstrate material. It is a requirement of the present invention thatthe nanowires produced by the processes described herein must projectout of the plane of the substrate. That is, the nanowires produced bythe processes of the present invention must extend off of the plane ofthe substrate material a distance greater than the dimension of a singlemolecule. As such, the nanowires produced according to the presentinvention are distinct from structures such as thin films and quantumdots, which spread on the surface of a substrate material, rather thangrowing in a manner such that they project out of the plane of thesubstrate a distance that exceeds the atomic diameter of a single Simolecule for instance.

Suitably, the nanowires produced according to any of the processes ofthe present invention will project out of the plane of the substratematerial so as to attain a final length of about 100 nm to less thanabout 1 μm. In suitable such embodiments, the nanowires producedaccording the present invention can attain a final length of a few 100nms. The nanowires of the present invention will suitably be at leastabout 1 nm to less than about 1 μm in diameter. For use in electronicdevices, the nanowires of the present invention will have a diameter ofabout a few nms to 100's of nms, so as to allow them to be harvested andutilized in an electronic device, suitably by placing the wires inplastic substrates so as to act as conductive media. (See U.S.Application No. 60/491,979, filed Aug. 4, 2003, for a description ofnanowire harvesting which is incorporated herein by reference.)

In suitable embodiments of the present invention, the nanowires, whengrowing on a crystalline substrate (whether polycrystalline or singlecrystal) will preferably grow in an epitaxial orientation. However, thepresent invention also embodies growth on crystalline substrates whereinthe nanowires do not grow in an epitaxial orientation. As used herein,the term epitaxial as it refers to the growth of nanowires means thatthe nanowires have the same crystallographic characteristic(s) as thesubstrate material on which they are growing. For example, theorientation of the substrate material can be any crystallographicorientation known to the ordinarily skilled artisan, including, but notlimited to, <111>, <110>, <100> and <211>. In suitable embodiments then,the nanowires produced by the processes of the present invention can begrown in any crystallographic orientation, and suitably in the sameorientation as the substrate material, including those orientationsdiscussed throughout and as known to the ordinarily skilled artisan.

As discussed throughout, suitable precursor gases that can be used inthe processes of the present invention comprise SiCl₄ and SiH₂Cl₂.Dissociation of these gasses generates free chloride ions (Cl) in thegas phase. These Cl ions, and/or additional chlorine ions introducedfrom a separate source of chlorine gas, act to etch the growing Sinanowires in an orientation that is preferentially a <111> orientation.

In other suitable embodiments of the present invention, thecrystallographic plane of the substrate material can be off axis of the0° horizontal plane. The nanowires growing on the surface of such asubstrate material can project out of the substrate material at an anglesuch that the wires can be normal to the crystallographic plane (i.e.,90° with respect to the crystallographic plane) or can be off axisrelative to the crystallographic plane such that they can be normal to a0° horizontal plane.

In embodiments of the present invention where amorphous substrates areutilized, the nanowires produced according to the processes of thepresent invention will not grow epitaxially, as the amorphous materialdoes not comprise a crystallographic orientation. However, as notedabove, the nanowires grown on such substrates may project out of theplane of the substrate at any angle relative to the horizontal plane.

The processes of the present invention produce nanowires that may carryelectrons between two points in space and thus act to transfer charge.In this way, the nanowires of the present invention are further distinctfrom nanodots and in their size and orientation, are distinct fromsemiconductor films.

In another embodiment, the present invention provides processes forproducing nanowires which does not require a starting metal catalyst, asoutlined in FIG. 3. FIG. 3 is a flowchart of a method for preparingnanowires using a combination of Si precursors which does not require astarting metal catalyst, according to an embodiment of the invention.

Method 300 begins in step 302. In step 302 a substrate material ishydroxylated to produce nucleation sites. In step 304 contacting thesubstrate material with a first precursor gas mixture generates one ormore nuclei on a surface of the substrate material, which is indicatedby label 306. In step 308 contacting the nuclei with a second precursorgas mixture, allows nanowires to grow at the site of the one or morenuclei, as indicated by label 310, until they reach the desired size andorientation, which is shown as step 314.

This process of the present invention does not require the use of ametal catalyst to provide a nucleation site for the nanowire, andtherefore eliminates the problems and concerns that arise due to metalsdiffusing into the growing nanowires. A similar process has beendescribed by De Salvo et al. for the production of nanocrystals in theform of nanodots (“How far will Silicon nanocrystals push the scalinglimits of NVMs technologies?,” IEEE Proceeding, Session 26, p. 1(2003)), but has not been extended to the production of nanowires as inthe present invention.

As discussed throughout, any suitable substrate material may be used forthe processes of the present invention. Suitably, the substrate materialwill be crystallographic, including both polycrystalline and singlecrystal substrates. In certain embodiments, the substrate materialutilized in this embodiment of the present invention will comprisesilicon. In other suitable embodiments of the present invention, thesubstrate material will be an amorphous material, including but notlimited to, SiO₂, Si₃N₄, or alumina.

Hydroxylation of the substrate material in these embodiments of thepresent invention can be generated by any suitable process known to theordinarily skilled artisan. For example, hydroxylation of a substratematerial of the present invention can be generated by chemical treatmentof the substrate material with diluted HF solution. Generation ofhydroxyl groups on the surface of the substrate material createnucleation points for Si or other semiconductor materials to deposit andinitiate nanowire growth.

Following hydroxylation, the substrate material is then contacted withone or more precursor gas mixtures to allow nucleation and initiation ofnanowire growth. Any precursor gas mixture known to the ordinarilyskilled artisan can be used in the processes of the present invention,and suitably can comprise dopants. Examples of precursor gases useful inthe practice of the present invention comprise, but are not limited to,SiH₄, Si₂H₆, SiCl₄ and SiH₂Cl₂, preferably SiH₄ or Si₂H₆, whichnucleates particles on the surface of the substrate, and in suitableembodiments may further comprise dopants such as, but not limited to,B₂H₆, TMB, POCl₃ and PH₃. The temperature for dissociation andnucleation of the nanowires is dependent upon the dissociationtemperature of the precursor gas mixture as discussed throughout. Insuitable embodiments, this temperature is at least about 300° C., but isoptimized based on the dissociation temperature of the precursor gasmixture as discussed throughout. In certain such embodiments, the firstprecursor gas mixture will comprise SiH₄.

Following nucleation and initiation of growth, the substrate material isthen contacted with one or more second precursor gas mixtures asdescribed throughout, and suitably can comprise SiH₄, Si₂H₆, SiCl₄ orSiH₂Cl₂, preferably SiCl₄ or SiH₂Cl₂, and may further comprise B₂H₆,TMB, POCl₃ or PH₃. In certain embodiments, the second precursor gasmixture will comprise SiCl₄ or SiH₂Cl₂. Use of such precursor gases willallow for growth in a <111> orientation when grown on a crystallographicsubstrate as a result of etching from the dissociated Cl as discussedabove. In other embodiments of the invention, as shown in step 312 ofFIG. 3, any number of precursor gases may be introduced to the nanowiresduring the initiation and growth processes, as long as one or more ofthe precursor gases is capable of nucleating particles on the surface ofthe substrate, and one or more precursor gas(es) aids in orienting thenanowires during the growth process (e.g., via etching). Providedfurther that as long as the temperature at which the wires contact themetal catalyst is above the dissociation temperature of the gas mixture,the wires will continue to grow. In other embodiments free H, Cl or Siatoms can be added to the growing nanowires as discussed throughout. Asdiscussed throughout, the processes of the present invention can be usedto produce nanowires that comprise various dopants and different regionsof these dopants throughout the length of the nanowire.

In embodiments of the present invention where crystallographicsubstrates are utilized, the nanowires produced will preferably grow inan epitaxial orientation, though the present invention also encompassesembodiments where growth on crystallographic substrates is notepitaxial. The processes of the present invention, as discussedthroughout, produce nanowires that grow and project out of the plane ofthe substrate material. As such, the nanowires of the present inventionare distinct from nanodots that do not grow in such an orientation, butrather remain in the plane of the substrate material. The presentinvention, by providing Si structures that project out of the plane ofthe substrate material, allow for the production of nanowires that canbe used to transport electrical charge as discussed throughout.

In an embodiment, a method for producing nanowires, includeshydroxylating a substrate material; contacting the substrate materialwith a first precursor gas mixture comprising a first precursor gas thatis capable of forming one or more nucleated particles on a surface ofthe substrate material; contacting the one or more nucleated particleswith a second precursor gas mixture, comprising at least one atomicspecies that aids in orienting the growing nanowires; and growingnanowires at the site of the one or more nucleated particles. In anaspect of this embodiment the first precursor gas mixture comprises SiH₄or Si₂H₆. In another aspect of this embodiment the second precursor gasmixture comprises SiCl₄ or SiH₂Cl₂. In another aspect of this embodimentthe second precursor gas mixture comprises chlorine gas from anindependent chlorine gas source which is separate from the source ofSiCl₄ or SiH₂Cl₂ gas. Nanowires can be produced by this method.Electronic circuits including nanowires produced by this method can alsobe produced.

The present invention also provides for nanowires produced by any of theprocesses of the present invention. As discussed throughout, nanowiresproduced by the processes of the present invention will suitably be of alength of at least about 100 nm and at least about 1 nm to less thanabout 1 μm in diameter, and may comprise various dopants (i.e., p- andn-type regions) throughout their length.

The present invention also provides for electronic circuits comprisingthe nanowires produced by any of the processes of the present invention.Suitably collections of nanowires produced according to the processes ofthe present invention are useful building blocks for high performanceelectronics. A collection of nanowires orientated in substantially thesame direction will have a high mobility value. Furthermore, nanowirescan be flexibly processed in solution to allow for inexpensivemanufacture. Collections of nanowires can be easily assembled onto anytype of substrate from solution to achieve a thin film of nanowires. Forexample a thin film of nanowires used in a semiconductor device can beformed to include 2, 5, 10, 100, and any other number of nanowiresbetween or greater than these amounts, for use in high performanceelectronics.

The nanowires of the present invention can also be used to make highperformance composite materials when combined with polymers/materialssuch as organic semiconductor materials, which can be flexibly spin-caston any type of substrate. Nanowire/polymer composites can provideproperties superior to a pure polymer materials. Further detail onnanowire/polymer composites is provided below.

Collections or thin films of nanowires of the present invention can bealigned into being substantially parallel to each other, or can be leftnon-aligned or random. Non-aligned collections or thin films ofnanowires provide electronic properties comparable or superior topolysilicon materials, which typically have mobility values in the rangeof 1-10 cm²/V·s.

Aligned thin films of nanowires of the present invention can be obtainedin a variety of ways. For example, aligned thin films of nanowires canbe produced by using the following techniques: (a) Langmuir-Blodgettfilm alignment; (b) fluidic flow approaches, such as described in U.S.Ser. No. 10/239,000, filed Sep. 10, 2002, and incorporated herein byreference in its entirety; and (c) application of mechanical shearforce. For example, mechanical shear force can be used by placing thenanowires between first and second surfaces, and then moving the firstand second surfaces in opposite directions to align the nanowires.Aligned thin films of nanowires/polymer composites can be obtained usingthese techniques, followed by a spin-casting of the desired polymer ontothe created thin film of nanowires. For example, nanowires can bedeposited in a liquid polymer solution, alignment can then be performedaccording to one of these or other alignment processes, and the alignednanowires can then be cured (e.g., UV cured, crosslinked, etc.). Analigned thin film of nanowires/polymer composite can also be obtained bymechanically stretching a randomly oriented thin film ofnanowires/polymer composite.

P-doped nanowires and n-doped nanowires produced by the processes of thepresent invention can be separately fabricated, and deposited in ahomogeneous mixture onto a surface, such as a macroelectronic substrate.On a macroscopic level, the resulting material appears to contain a highconcentration of both n- and p-dopants. By creating such a mixture of p-and n-doped nanowires, macroelectronic devices can be fabricated thatrespond as if they are both n- and p-doped. For example, a resultingthin film of nanowires that includes both n-doped and p-doped nanowirescan exhibit characteristics of both n-doped and p-doped nanowires. Forexample, diode, transistor, and other known electrical devices can befabricated to include a combination of p-doped nanowires and n-dopednanowires.

Nanowires produced by the processes of the present invention can also beused to produce electrical devices such as p-n diodes, transistors, andother electrical device types, using nanowire heterostructures asdescribed herein. Nanowire heterostructures include a plurality of p-njunctions along the length of the nanowire and can include alternatingportions or segments along their lengths that are differently doped.

Methods for Nanowire Synthesis

FIG. 4 is a flowchart of method 400 for nanowire synthesis, according toan embodiment of the invention. Method 400 provides a method forsynthesis of nanowires using synthesis vessel 455, as illustrated inFIG. 4A. In one embodiment, synthesis vessel 455 is a sealed quartztube. Synthesis vessel 455 can be, but is not limited to, tubes,pipettes, and the like. The dimensions of synthesis vessel 455 can bevaried to meet the particular production needs of the user. In researchscale operation, the dimensions for synthesis vessel 455 typically rangefrom six to eighteen inches in length and one-half inch to one inch indiameter. Method 400 begins in step 410. In step 410, a granularprecursor material is position at one end of a synthesis vessel, whichis at temperature T1. For example, referring to FIG. 4A, precursormaterial 470 is positioned at end 460 of synthesis vessel 455. Examplegranular precursor materials can include, but are not limited to Si, CdSand GaN. T1 will be a function of the type of granular precursormaterial. For example, if the granular precursor material is CdS, thenT1 can be in the range from about 800 to 950 degrees centigrade. In thisexample, T2 would be approximately 50 degrees centigrade less than T1with some variance based on the type of transport mechanism andtransport agent, as would be known by individuals skilled in therelevant arts based on the teachings herein.

In step 420, catalyst particles supported by a substrate are positionedat the other end of a synthesis vessel, which is at temperature T2. Forexample, referring to FIG. 4A, catalyst particles supported by substrate470 are positioned at end 465 of synthesis vessel 455. In exampleembodiments, catalyst particles can include Au when Si is used as thegranular precursor material, and Ni when GaN is used as the granularprecursor materials.

In step 430, materials are transported from one end of the synthesisvessel to the other. In one embodiment, thermal vaporization is used totransport the granular precursor materials, such as granular precursormaterials 470 to the other end of the vessel. In another embodiment, achemical vapor transport agent can be used to transport the granularprecursor material. Chemical vapor transport agents can include, but arenot limited to chlorine, iodine, and bromine.

In step 440, a transfer agent is reacted with the granular precursormaterials to form nanowires. In particular, the transfer agent reactswith granular precursor materials at T1 to form a volatile compound,which will diffuse to the surface of catalyst particles and decompose togrow nanowires at T2. At the same time, the transport agent isregenerated and ready for another cycle to produce more nanowires. Thegrowth cycle will continue until the process is terminated.

Methods for Nanowire Doping

FIG. 5 is a flowchart of method 500 for doping nanowires, according toan embodiment of the invention. Method 500 is similar to method 400,except that instead of synthesizing nanowires as was done in method 400a similar approach to that of method 400 is used to dope nanowires.Method 500 begins in step 510. In step 510, a dopant precursor materialis positioned at one end of a synthesis vessel, which is at temperatureTi. Example dopant precursor materials can include, but are not limitedto Mg, B, and PO₄.

In step 520, nanowires are positioned at the other end of a synthesisvessel, which is at temperature T2. Example nanowires can include, butare not limited to GaN nanowires and Si nanowires. In step 530,materials are transported from one end of the synthesis vessel to theother. In one embodiment, thermal vaporization is used to transport thedopant precursor materials to the other end of the vessel. In anotherembodiment, a chemical vapor transport agent can be used to transportthe dopant precursor material. Chemical vapor transport agents caninclude, but are not limited to, chlorine and bromine. When phosphatesare used as the precursor dopant material, the phosphate can also serveas the chemical vapor transport agent.

In step 540, the transfer agent is reacted with the granular precursormaterials to form nanowires. In particular, the transfer agent reactswith granular precursor materials at T1 to form a volatile compound,which will diffuse to the surface of the nanowires and decompose to grownanowires at T2. At the same time, the transport agent is regeneratedand ready for another cycle to dope more nanowires. The growth cyclewill continue until the process is terminated.

The above method can be summarized as a method for doping nanowires,including positioning a granular dopant precursor material at one end ofa vessel at a first temperature; positioning nanowires at an oppositeend of the vessel at a second temperature; transporting granular dopantprecursor materials from one end of the vessel to the other end of thevessel; and reacting granular dopant precursor material with nanowiresto form doped nanowires. Thermal vaporization can be used to transportthe granular nanowire precursor material. A chemical vapor transportagent can be used to transport the granular nanowire precursor material.The chemical vapor transport agent can be one of chlorine, iodine andbromine. The granular dopant precursor material can be Mg and thenanowires can be GaN. The nanowires can be Si and the granular dopantprecursor material can be B.

Methods to Improve Nanowire Quality

FIG. 6 is a flowchart of method 600 to reduce surface states fromdangling bonds on a nanowire structure, according to an embodiment ofthe invention. In the growth of nanowires, such as Si nanowires,dangling Si bonds often form at the interface between a Si nanowire anda dielectric that inhibit nanowire performance. Hydrogen passivation isoften used in Si nanowire processing to reduce surface states fromdangling bonds to address this problem. However, the hydrogen plasma caninduce surface damage during processing. Method 600 describes anapproach to use a sacrificial layer to protect the nanowire, whilepassivation is used to reduce surface states of dangling bonds.

Method 600 begins in step 610. In step 610 a nanowire structure iscreated. In step 620 a sacrificial layer is deposited on the nanowirestructure. For example, FIG. 7 is a diagram of a nanowire structurecoated with a sacrificial layer, according to an embodiment of theinvention. The nanowire structure includes nanowire 710, dielectric 720,gate 730 and sacrificial layer 740. Dielectric 720 coats nanowire 710,while gate 730 coats dielectric 720. Sacrificial layer 740 coats gate730. The entire nanowire structure is formed on substrate 750. In analternative embodiment, the nanowire structure can include only ananowire, such as nanowire 710, which is formed on substrate 750.Sacrificial layer 740 can include, but is not limited to SiO₂ or SiN.Dielectric 720 can include, but is not limited to SiO₂ or SiN. Gate 730can include doped amorphous Si, but is not limited to doped amorphousSi. Moreover, substrate 750 can include Si, but is not limited to Si.

In step 630 the nanowire structure with the sacrificial layer, such assacrificial layer 640, is passivated. In an embodiment, hydrogenpassivation can be used. In step 640 the sacrificial layer is chemicallyremoved. In an embodiment, selective etching with a wet etchant can beused to chemically remove the sacrificial layer, as would be known byindividuals skilled in the relevant arts based on the teachings herein.The particular wet etchant to use would be selected based on theparticular material contained within the sacrificial layer and the typeof nanowire.

Methods for Modifying Nanowires Directly on a Substrate for DevicePreparation

In another embodiment, the present invention provides processes by whichnanowires can be modified directly on a substrate for devicepreparation. Preferably, the nanowires used in these processes will besubstantially vertical nanowires. Vertical nanowires encompass nanowiresthat are substantially perpendicular to the surface of the substrate onwhich they are grown or deposited. Suitably, the vertical nanowires willbe oriented such that they are between about 45° and about 90° withrespect to the horizontal plane of the substrate, more suitably about60° to about 90°, and most suitably about 80° to about 90° with respectto the horizontal plane of the substrate. Such nanowires can be producedusing any suitable nanowire growth process known in the art, includingthose disclosed herein. While any substrate material disclosed hereincan be used as a nanowire growth platform, suitably, the substratematerial will be single-crystalline or polycrystalline, such that growthfrom the substrate will generate oriented, straight, single crystaldimension wires (suitably epitaxially oriented nanowires). In otherembodiments, the nanowires can be horizontal, such as disclosed in U.S.Provisional Patent Application No. 60/632,337, filed Dec. 2, 2004, thedisclosure of which is incorporated by reference herein in its entirety.In further embodiments, after processing of the nanowires on the growthsubstrate, the nanowires can be removed from the substrate by coatingthe nanowires with a polymer to form a composite, and then removing thenanowires from the substrate.

In one embodiment, the present invention provides processes forproducing a nanowire device, comprising: providing a substrate havingnanowires attached to a surface in a vertical orientation; depositing adielectric layer on the surface of the nanowires; depositing one or morenanowire contacts on the nanowires; depositing a material over thenanowires to form a nanowire composite; and separating the nanowirecomposite from the substrate. The nanowire composite can then betransferred to a second substrate where the nanowires can be metallized.

Any on-substrate processing known or required by those skilled in theart can be performed on the substantially vertical nanowires. Byproviding nanowires that are separate, oriented and vertical, deviceprocessing of the wires is simplified and improved. In certainembodiments, a dielectric layer can be grown or deposited directly onthe wires. The dielectric layer can be formed by oxidizing thenanowires, or otherwise forming the dielectric layer. Polymerdielectrics for use in the present invention include for example,polyimides, fluorinated polyimides, polybenzimidazoles and others.Dielectrics for use in the invention include SiO₂, Ta₂O₅, TiO₂, ZrO₂,HfO₂, Al₂O₃, and similar materials. Nitride dielectrics include AlN andSiN. As the wires are substantially separate, each wire can be fullycoated with a dielectric material without the concern of sections ofoverlapping wire not receiving coating. In addition, further processingcan include oxidation or nitridization of the nanowires. Nitridation ofnanowires can be accomplished with processes similar to those employedin oxidation of nanowires. These materials can be applied to nanowiresby chemical vapor deposition (CVD), solution phase over-coating, or byspin-coating the appropriate precursor onto the substrate. Other knowntechniques can be employed, for example sputtering and others.

In other embodiments, an implant, such as a gate electrode, can be addedto the nanowires. Nanowire contacts, including sources, drains andgates, for example, can be pattered on the substrate using standardphotolithography, ink-jet printing, or micro-contact printing processes,for example, or by other processes as described throughout.

FIG. 8 is a diagram showing nanowire processing in accordance with oneembodiment of the present invention. As shown in FIG. 8, substantiallyvertical nanowires 804 can have dielectric layers 812 and/or metalliccontacts 808, for example, deposited onto them. The nanowires can thenbe encapsulated in polymer 806, thereby producing a composite comprisingpolymer and nanowires. By covering all or a portion of the nanowires,composites comprising various densities of nanowires can be produced.Vertically grown nanowires will suitably be of the same overall length,owing to control of growth times and conditions. In addition, furtheralignment of the nanowires may not be necessary, as the verticalnanowires will already be substantially aligned and oriented when grown,especially when grown from a poly- or single-crystalline substrate 802.Creating a polymeric composite allows several nanowires to be removedtogether and subsequently transferred to a second, third, forth, etc.substrate material where metallic contacts can be added or additionalprocessing can take place.

Suitable polymers for use in forming the nanowire composites include,but are not limited to, elastomers, thermoplastics and thermosettingresins. Useful polymers include oligomers, which includes, but is notlimited to, monomers, dimers, trimers, tetramers, pentamers, hexamers,heptamers, octamers, nonamers, decamers, undecamers, and dodecamers;branched, hyperbranched, dendritic and other non-linear structural formsof polymers; prepolymers such as phenoxy and epoxy prepolymers;networked polymers such as interpenetrating and semi-interpenetratingnetwork polymers; homopolymers, copolymers, terpolymers and othercopolymers including random, statistical, alternating, block and graftcopolymers and blends of two or more different polymers. Particularexamples of polymers for use in nanowire composites include, but are notlimited to, polyalkanes, polyhaloalkanes, polyalkenes, polyalkynes,polyketones, polycarbonates, polyamides, polyimides, polyarylenes,polyarylvinylenes, polyheteroarylenes, polyheteroarylvinylenes,polyesters, polyethers, polyurethanes, polybenzimidazoles, polysulfides,polysiloxanes, polysulfones, polysaccharides, polypeptides,polyphosphazenes, polyphosphates, phenolic and phenol-formaldehyderesins, epoxy and phenoxy resins, and urea- and melamine-formaldehyderesins. Such composites and methods for their preparation are disclosedin U.S. patent application Ser. No. 10/910,800, filed Aug. 4, 2004,which is incorporated herein by reference in its entirety.

In suitable embodiments, the polymer composite will comprise a mixtureof at least two or more different polymers, and in certain embodiments amixture of at least two epoxy polymers.

Removal of the polymer-nanowire composite 810 from substrate 802 can beachieved via any suitable method, including mechanical separation (e.g.,cutting with a blade or tearing the nanowires from the substrate),chemical separation, including wet or dry etching, orchemical-mechanical polishing. Suitable etching agents include, but arenot limited to KOH, XeF₂ and hydrofluoric acid and can be combined withan oxidizing agent such as HNO₃ or H₂SO₄. In other embodiments of thepresent invention, a removable layer can separate the nanowires from thesubstrate material. After generating the polymer-nanowire composite, theremovable layer can be removed. For example, the removable layer can bedissolved by a solvent that will dissolve the removable layer, but notdissolve the polymer-nanowire composite or the substrate below theremovable layer. In other embodiments, the removable layer can be aphotoremovable layer, in which ultraviolet or other suitable wavelengthsof light are used to remove the removable layer, thereby allowingseparation of the polymer-nanowire composite from the substrate. Oneexample of a photoremovable layer is a substance that breaks down anddisintegrates in the presence of ultraviolet light. Suitably, thecomposite material used to encapsulate the nanowires will be hardenedprior to removing the nanowires from the substrate.

Following removal of the polymer-nanowire composite, the composite canthen be transferred to a second substrate where further processing cantake place. For example, this second substrate can be selected so as totolerate elevated temperatures, such that doping of the nanowires cantake place. Methods by which to dope the nanowires are well known in theart and can be used to dope the nanowires and polymer-nanowirecomposites of the present invention.

FIG. 9 is a diagram showing nanowire processing following transfer inaccordance with one embodiment of the present invention. As shown inFIG. 9, the polymer-nanowire composites 810 can subsequently betransferred to an additional substrate 902 where the nanowires can bemetallized 904 to form electrical conductivity to device regions (e.g.,gain, source, gait). In such embodiments, nanowires 804 can be coupledbetween a source electrode 906 and a drain electrode 908 over a portionof the gate electrode 910. In other embodiments, source and drainelectrodes can be added and ohmic contacts can be generated on thewires. As the wires are further “anchored” by the metal contacts, gateisolation and metal processing steps as known in the art can be used tofinalize the nanowire preparation. Such processing allows for wafersthat can comprise multiple semiconductor devices on the same basesubstrate. In other embodiments, such processing can occur directly onthe growth substrate 802, followed by removal of the nanowire composite,such that all, or substantially all, nanowire processing is prepared onthe initial growth substrate 802.

Semiconductor devices prepared according to the present invention canhave any number of contact areas formed to provide electricalconnectivity. For example, anode and cathode electrodes can be formedwith diodes or other two-terminal devices. Other semiconductor devicescan have greater or fewer numbers of contact areas formed. Suitablemethods of preparing semiconductor devices using the processes andnanowires of the present invention are disclosed in U.S. patentapplication Ser. No. 10/674,060, filed Sep. 30, 2003, which isincorporated herein by reference in its entirety.

A variety of contact area types can be formed according to the presentinvention. The contact areas can be Ohmic and non-Ohmic. For example, anon-Ohmic Schottky diode barrier contact can be used as an electrode. ASchottky diode barrier contact is commonly used for a III-Vsemiconductor material when it is difficult to make high quality gatedielectrics. Source electrodes, gate electrodes, and drain electrodesare formed of a conductive material, such as a metal, alloy, silicide,polysilicon, or the like, including combinations thereof, as would beapparent to a person having ordinary skill in the art. Note that theorder in which the gates, sources, and drains are patterned can bevaried. For example, gates, sources, and drains can be patternedsimultaneously with each other or at different times.

Making reliable ohmic contacts with nanowires is difficult due to smallcontact areas and complicated interface states. Interfacial chemistryand physics between metal contacts and silicon are important technicalareas regarding ohmic contacts. A key to success is the precise controlof the metallization process and surface cleaning process prior tometallization. Suitable metallization schemes include Ti—Au, Ni and Alby electron beam (e-beam) evaporation. Various further processes,including ion gun cleaning, or HF etching can be employed to remove thesurface dielectrics prior to metallization of source-drain electrodes.

In an embodiment of the invention a method for producing a nanowiredevice, includes providing a substrate having nanowires attached to asurface in a vertical orientation with respect to the substrate;depositing a dielectric layer on the surface of the nanowires;depositing one or more nanowire contacts on the nanowires; depositing amaterial over the nanowires to form a nanowire composite; and separatingthe nanowire composite from the substrate. This method can furtherinclude transferring the nanowire composite to a second substrate. Thismethod can further include metallizing the nanowires. In this method thesubstrate can be crystalline or polycrystalline. In this method, thematerial can a polymer, which include an elastomer, thermoplastic orthermosetting resin. In this method the material can include a mixtureof at least two or more different polymers. In this method the materialcan include a mixture of at least two epoxy polymers. In this method,the material can be hardened after depositing a material over thenanowires to form a nanowire composite and before separating thenanowire composite from the substrate. In this method separating thenanowire composite from the substrate includes using a blade orchemically etching the nanowire composite from the substrate.

Methods to Harvest Nanowires

FIG. 10 is a flowchart of method 1000 for harvesting a nanowire using asacrificial portion, according to an embodiment of the invention.Nanowires are presently grown on substrates using an catalytic colloidto initiate nanowire growth. A difficulty limiting the ability toconsistently grow and harvest high quality nanowires is the ability toremove the nanowires from the substrate without incurring physicaldamage to the wire, such as breakage. Current removal methods includeusing ultrasound and mechanical shearing. However, a problem with eachof these is that they cause random breaks in the nanowires resulting ina non-uniform distribution of nanowire lengths.

Methods 1000, 1200, 1300, 1500 and 1600 provided below describe methodsto address this problem. In particular, method 1000 involves the use ofa sacrificial portion of a nanowire to provide more effective removal ofthe nanowires.

Method 1000 begins in step 1010. In step 1010 a desired portion of ananowire is grown. In one embodiment, an Au colloid is used to grow thedesired portion of the nanowire. In step 1020 a sacrificial portion of ananowire is grown that has different properties from the desiredportion. FIG. 11 illustrates a nanowire with a desired and sacrificialportion of a nanowire, according to an embodiment of the invention. Inparticular, FIG. 11 shows a nanowire including three parts—sacrificialportion 1110, desired portion 1120 and stub 1130, which is describedbelow, that has been grown on substrate 1140.

In an embodiment, the properties that differ can be that differentalloys are used for the sacrificial and desired portions of thenanowire. For example, the sacrificial portion can be SiGe and thedesired portion can be Si.

In another embodiment, doping can be varied for the sacrificial anddesired portion of the nanowire. For example, the sacrificial portioncan be n-doped and the desired portion can be p-doped. Alternatively,the sacrificial portion can be p-doped and the desired portion can ben-doped. Similarly the sacrificial portion can have no doping, and thedesired portion can be either p-doped on n-doped.

In an embodiment where different alloys are used for the desired andsacrificial portions of the nanowire, an Au alloy can be used to growthe sacrificial portion of the nanowire. The Au alloy can include, butis not limited to AuGe or AuSiGe, when Si nanowires are being grown andharvested. In an embodiment, the sacrificial portion of the nanowire caninclude SiGe and the desired portion can include Si. In this embodiment,Au would be used in step 1010 and AuGe would be used in step 1020 tostimulate nanowire growth, for example.

In an embodiment where the sacrificial portion of the nanowire isp-doped and the desired portion of the nanowire is n-doped, a boron NWScan be used as a dopant during the growth of the sacrificial portion ofthe nanowire.

In step 1030 the desired portion of the nanowire is protected. In oneexample, a photoresist can be applied to the desired portion of thenanowire to protect the nanowire. In an alternate embodiment the desiredportion of the nanowire is not protected.

In step 1040 the sacrificial portion of the nanowire is differentiallyremoved. The sacrificial portion of the nanowire can be differentiallyremoved by using a wet etchant with a etching rate that is significantlyhigher for the materials within the sacrificial than for the materialswithin the desired portions of the nanowire. For example, hydrofluoricperoxide acetic acid (1HF:2H₂O₂:3CH₃COOH) can be used to remove asacrificial portion that contains SiGe, when the desired portion is Si.When using this etchant, the etchant removes the SiGe alloy and stopsefficiently at the Si surface of the desired portion. Other etchants canbe used, as would be known by persons skilled in the relevant arts basedon the teachings herein.

In an alternate embodiment, where the sacrificial portion of thenanowire is p-doped using a boron NWS as a dopant and the desiredportion of the nanowire is n-doped example etchants can include, by arenot limited to potassium hydroxide (KOH), tetramethylammonium hydroxide(TMAH) and ethylene diamine/pyrocatechol/water (EDP). These etchantsetch the sacrificial portion of the nanowire at a rate ranging from 27:1to greater than 500:1 compared to the etch rate of the desired portion,when Si nanowires are used, for example. The range depends on thespecific etchant and the temperature, as would be known by individualsskilled in the art.

In step 1050 a stub at the tip of the nanowire is removed. Typically,this stub will be a residual and undesirable by-product of the catalystused to initiate nanowire growth. Methods of removal will be known byindividuals skilled in the relevant arts. In step 1060 method 1000 ends.

FIG. 12 is a flowchart of method 1200 for harvesting a nanowire whilemonitoring of a PN junction, according to an embodiment of theinvention. Method 1200 is similar to the embodiment involving differentdoping levels described with respect to method 1000. In method 1200,however, leakage current across a PN junction created between asacrificial and desired portion of the nanowire is monitored todetermine when the sacrificial portion has been successfully etchedaway.

Method 1200 begins in step 1210. In step 1210 a desired portion of ananowire is grown. In step 1220 a sacrificial portion of the nanowire isgrown. The desired portion is grown, such that the desired portion isdifferentially doped from the sacrificial portion. In step 1230 thedesired portion of the nanowire is protected. In an embodiment, aphotoresist can be applied to the desired portion of the nanowire. Instep 1240 the sacrificial portion of the nanowire is differentiallyetched in a manner similar to that described with respect to step 1040of method 1000. In step 1250 the leakage current between a PN junctionbetween the desired portion and sacrificial portion of the nanowire ismonitored. Steps 1240 and 1250 occur simultaneously. In step 1260 whenthe leakage current suddenly increases indicating that the sacrificialportion has been successfully etched away, etching is stopped. In step1270 method 1200 ends.

FIG. 13 is a flowchart of method 1300 for harvesting a nanowire using asacrificial metal layer on a nanowire growth substrate, according to anembodiment of the invention. FIG. 14 illustrates, for example, thegrowth of nanowires, such as nanowire 1410, on sacrificial metal layer1430. Sacrificial metal layer 1430 is layered on top of a silicon oxidelayer 1420.

Method 1300 begins in step 1310. In step 1310 a nanowire growthsubstrate is selected. In step 1320 an oxide or nitride layer, such assilicon oxide layer 1320, is placed on the nanowire growth substrate. Instep 1330 a metal layer, such as metal layer 1430, is placed on thenitride or oxide layer. The metal layer can include, but is not limitedto Au, Al, Ti, or Cr.

In step 1340 nanowires are grown on the surface of the metal layer, aswould be known by individuals skilled in the relevant arts based on theteachings herein. In step 1350 the metal layer is removed. In oneembodiment, a metal etchant is used that removes the metal layer, butdoes not affect the nanowire. In step 1360 the nitride or oxide layer isremoved. Similar to the case of the removing the metal layer, an etchantis used that removes the nitride or oxide layer without affecting thenanowire. By removing this layer the nanowires are released and can beharvested. In step 1370 method 1300 ends.

FIG. 15 is a flowchart of method 1500 for harvesting a Si nanowire whenusing a non-Si substrate to grow the Si nanowire, according to anembodiment of the invention. Method 1500 begins in step 1505. In step1505 a non-Si substrate is selected. Example materials that can be usedas the non-Si substrate include high temperature metals, Ge, and quartz.In step 1510 Si nanowires are grown on the non-Si substrate. In step1520 the Si nanowire are protected. In an embodiment, the Si nanowiresare coated with a photoresist. In alternative embodiments, the Sinanowires are not protected. In step 1530 the non-Si substrate isselectively wet etched to release the Si nanowires. In step 1540 method1500 ends.

FIG. 16 is a flowchart of method 1600 for harvesting a nanowire with oneorientation when a nanowire growth substrate with a differentorientation is used, according to an embodiment of the invention. FIG.17 provides a diagram of nanowire 1720 with one orientation growing onnanowire growth substrate 1710 with a different orientation, accordingto an embodiment of the invention. FIG. 17 shows portion 1730 ofnanowire growth substrate 1710 that is etched away to free nanowire1720. In an embodiment, nanowire 1720 can be Si, with atoms orientedsuch that the Miller indices are <111>. Nanowire growth substrate canalso be Si with atoms oriented such that the Miller indices are <100>.In an alternative embodiment Si atoms can have Miller indices of <111>within nanowire 1710 and the Miller indices of Si atoms within the Sinanowire growth substrate can be <110>.

Method 1600 begins in step 1610. In step 1610 a nanowire growthsubstrate is selected that has a first orientation. For example,nanowire growth substrate 1710 with either a <100> or <110> orientationcan be used. In step 1620 nanowires are grown on the nanowire growthsubstrate. The nanowires grown in step 1620 have a different orientationthan the orientation of the nanowire growth substrate. For example,nanowire 1720 with a <111> orientation can be grown. In step 1630 thenanowires are protected. In an embodiment a photoresist can be appliedto nanowires, such as nanowire 1720. In step 1640 the nanowire growthsubstrate is differentially etched to free the nanowires. For example,in an embodiment a wet etchant, including but not limited to KOH or TMAHcan be used to differentially etch nanowire growth substrate 1610. Instep 1650 method 1600 ends.

In another embodiment of the invention, a method of harvesting ananowire includes growing a desired portion of the nanowire; growing asacrificial portion of the nanowire with different properties from thedesired portion of the nanowire; differentially removing the sacrificialportion of the nanowire; and removing a growth stub from the desiredportion of the nanowire. This method can further include protecting thedesired portion of the nanowire. In this method an Au alloy, such as,for example, AuGe or AuSiGe, can be used to grow the sacrificial portionof the nanowire. In this method the sacrificial portion of the nanowirecan include SiGe and the desired portion can include Si. In this methoddifferentially removing the sacrificial portion of the nanowire canfurther include using a wet etchant to selectively chemically etch toremove the sacrificial portion of the nanowire. In an embodiment, thewet etchant can be Hydroflouric Peroxide Acetic Acid(1HF:2H₂O₂:3CH₃OOH).

In another embodiment of the invention, a method of harvesting ananowire includes growing a sacrificial portion of the nanowire that isn- or p-doped; growing a desired portion of the nanowire that is notdoped or oppositely doped from the sacrificial portion of the nanowire,whereby a PN junction is created within the nanowire at a junctionbetween the sacrificial portion of the nanowire and the desired portionof the nanowire; differentially etching the sacrificial portion of thenanowire; monitoring the leakage current at a PN junction between thesacrificial portion of the nanowire and the desired portion of thenanowire; and stopping etching when a sudden increase in leakage currentacross the PN junction occurs.

In an embodiment of the invention, a method for harvesting a nanowire,includes establishing a nanowire growth substrate; forming a nitride oroxide layer on the nanowire growth substrate; forming a metal layer ontop of oxide or nitride layer; growing the nanowire; removing the metallayer; and removing the oxide or nitride layer to free the nanowire. Inthis method, the metal layer can be formed using Au, Al, Ti, or Cr. Inthis method a metal etchant is used that does not etch the nanowires. Inthis method when removing the oxide or nitride layer, an etchant is usedthat does not etch the nanowires.

In an embodiment of the invention, a method of harvesting a nanowire ofa first material, includes establishing a substrate of a secondmaterial; forming the nanowire of the first material on the substrate ofthe second material; protecting the nanowire of the first material; andselectively wet etching the substrate of the second material to removethe nanowire of the first material. In this method the first materialcan be Si and the second material can be a high temperature metal. Inthis method the second material can be germanium. In this method thenanowire can be Si and a SiGe stub is formed at growth initiation tocontrol a length of the nanowire after etching. In this method thesecond material can be quartz.

In an embodiment of the invention, a method of harvesting a nanowirewith a first material with a first orientation, includes establishing asubstrate of a second material with a second orientation; growing thenanowire of the first material with the first orientation on thesubstrate of a second material with the second orientation; protectingthe nanowire of the first material with the first orientation; andselectively wet etching based on orientation the substrate of the secondmaterial with the second orientation to free the nanowire of the firstmaterial with the first orientation. In this method the first materialcan be Si and the second material can be Si and the first crystalorientation is <111> and the second orientation is <100>. In the methodselectively wet etching the substrate of the second material with thesecond orientation includes using Potassium Hydroxide (KOH) ortetramethylammonium hydroxide (TMAH).

Methods for Transferring Nanowires from a First Substrate to a SecondSubstrate Using a Teflon-Like Coated Surface

FIG. 18 is a flowchart of method 1800 for transferring nanowires from afirst substrate to a second substrate, according to an embodiment of theinvention. FIGS. 20A and 20B will be referred to during the descriptionof method 1800. FIG. 20A is a diagram of first substrate 2010 withnanowires 2020 and a transfer substrate 2040 with a non-stick coating2030, according to an embodiment of the invention. FIG. 20B is a diagramof device substrate 2050 and transfer substrate 2040 with non-stickcoating 2030 for transferring nanowires 2020, according to an embodimentof the invention. Device substrate 2050 contains nanowire placementareas, such as nanowire placement area 2060 where nanowires are to belocated. In other embodiments, nanowires can be placed all over devicesubstrate 2050.

Referring again to FIG. 18, method 1800 begins in step 1810. In step1810 a transfer surface is coated with a non-stick coating, such asTEFLON. For example, transfer substrate 2040 can be coated withnon-stick coating 2030. In other embodiments, TEFLON-like, which havenon-stick surfaces can be used.

In step 1820 the transfer surface with the non-stick coating is pressedagainst nanowires that are affixed to a first substrate. Sufficientpressure is applied to affix the nanowires to the non-stick coating andremove them from the first substrate. For example, transfer substrate2040 with non-stick coating 2030 can be pressed against nanowire growthsubstrate 2010 to remove nanowires 2020.

In step 1830 the transfer substrate is positioned above the secondsubstrate. For example, referring to FIG. 20B, transfer substrate 2040with non-stick coating 2030 containing nanowires 2020 is placed abovedevice substrate 2050. In step 1840 pressure is applied to the backsideof the transfer surface to release the nanowires. In one embodiment,pressure is applied uniformally on the backside of a transfer surface,such as transfer substrate 2040. In another embodiment, pressure can beapplied in a patterned fashion to match the areas where nanowires are tobe placed on a second substrate. For example, pressure can be applied tothe backside of transfer substrate 2040 only above the nanowireplacement areas 2060 to release nanowires that will then be positionedwithin those areas. In step 1850 method 1800 ends.

FIG. 19 is a flowchart of method 1900 for transferring nanowires from afirst substrate with a patterned coating to a second substrate,according to an embodiment of the invention. Method 1900 is similar tomethod 1800, except that the non-stick coating is only applied tocertain areas on a transfer substrate that would correspond to nanowireplacement areas on a second substrate where the nanowires are to betransferred.

Method 1900 begins in step 1910. In step 1910 a transfer surface ispatterned with a non-stick coating, such as TEFLON or a TEFLON-likematerial. As stated above the patterned area can correspond to nanowireplacement areas on the second substrate where the nanowires are to bedeposited.

In step 1920 the transfer surface is pressed against nanowires affixedto a nanowire growth substrate. In step 1930 the transfer surface ispositioned above a second substrate. In step 1940 pressure is applieduniformly to the backside of the transfer surface to release thenanowires. In an alternative embodiment, pressure can be applied only tothe patterned areas of the transfer surface. In step 1950 method 1900ends.

Methods for Transferring Nanowires from a First Substrate to a SecondSubstrate Using a Thermal Press Technique

In another embodiment, the present invention provides methods fortransferring nanowires from a growth substrate to a transfer substrate.For example, these embodiments are useful to transfer nanowires totransfer substrates that are suitably flexible, polymeric, materials. Asnoted throughout, it is a desire of nanowire processing to generatesubstantially oriented, separate nanowires that can then be furtherprocessed for use as electronic devices. In this embodiment, nanowirescan be oriented individually during the transfer process, or the wirescan be oriented prior to transfer, and then transferred as a whole toallow for easier device processing.

FIG. 21 is a representation of probe nanowire transfer scheme inaccordance with one embodiment of the present invention. In oneembodiment, illustrated in FIG. 21, the present invention providesprocesses, for example as shown in transfer scheme 2100, fortransferring nanowires from a growth substrate 2102 to a secondsubstrate 2106 with the use of pressure applied via probe 2108.Nanowires can be grown using any suitable method known in the art,including those described herein. As shown in FIG. 21, nanowires 2104are grown on substrate 2102. In addition to substrate 2102, theapparatus used for nanowire growth can further comprise removable layersor additional separation layers between substrate 2102 and nanowires2104. Nanowires 2104 are transferred from the surface of substrate 2102onto transfer substrate 2106 by applying pressure with probe 2108. Anytransfer substrate can be used in the practice of the present invention.Suitably, transfer substrate 2106 will be a flexible polymeric sheet orpolymeric film such as a film of polyethylene terephthalate (PET).Additional polymers that can be used as transfer substrate 2106 include,but are not limited to, thermoplastics, such as polyethylenes,polypropylenes, polystyrenes, polysulphones, polyamides, polycarbonates,thermosetting resins, elastomers and the like. The flexibility oftransfer substrate 2106 can vary between a rather stiff, yet deformablematerial, and a highly malleable material. The amount of heat (seebelow) and pressure required to transfer nanowires to the transfersubstrate depends upon the choice of transfer substrate. For example, ifa malleable transfer substrate is selected, only a moderate amount ofheat may be required to make the surface of the transfer substratetacky. If however, a stiffer transfer substrate is selected, a higheramount of heat my be required to not only make the substrate surfacetacky, but also to allow it to be malleable enough so that it can bedeformed and make contact the substrate and nanowires in desired areas.

Probe 2108 will suitably be on the order of about millimeters to aboutcentimeters in diameter at probe tip 2110, and generally will be in aconical or needle-like shape, though any suitable shape can be used.Probe 2108 and probe tip 2110 can be made from any suitable materialthat will withstand the applied pressure (and heat if required),including polymers, plastics and metals. In certain embodiments, theaddition of pressure at probe tip 2110 is sufficient to transfernanowires 2104 from substrate 2102 onto transfer substrate 2106. Inother embodiments, an adhesive can be applied to the surface of transfersubstrate 2106 so that nanowires 2104 will adhere to, and remain adheredto, transfer substrate 2106 following application of pressure by probe2108. Suitable adhesives that can be used to coat the transfersubstrates include, but are not limited to polyimides, epoxies, andother polymeric adhesives.

In additional embodiments, probe 2108 and probe tip 2110 can be heatedso that the surface of transfer substrate 2106 will slightly melt,thereby becoming tacky or sticky, such as to act like an adhesive. Insuch embodiments, probe 2108 and probe tip 2110 are suitably made frommetal that can withstand the applied heat. The temperature required toheat probe 2108 is dependent upon the temperature at which transfersubstrate 2106 becomes tacky or sticky, but should not be so high thattransfer substrate 2106 deforms excessively or flows under the appliedpressure. Suitably, this temperature will be about 40° C. to about 100°C., depending upon the material selected as transfer substrate 2106.Suitably, when using PET as the transfer substrate, the temperature usedwill be about 60° C. The amount of pressure that is applied to probe2108 and probe tip 2110 is largely dependent upon the flexibility andstability of transfer substrate 2106. The pressure should be such thatthe substrate is brought in contact with nanowires 2104 only in areaswhere nanowire transfer is desired. In suitable embodiments, thepressure applied to probe 2108 and probe tip 2110 will be on the orderof about 10's of pounds per square inch (psi).

Application of pressure to nanowires 2104, in conjunction with a heatedprobe tip 2110 allows nanowires 2104 to transfer from substrate 2102onto transfer substrate 2106 and remain there as the pressure and/orheat is reduced. Using the processes of the present invention, nanowirescan be individually aligned on transfer substrate 2106 by selectivelyapplying heat and/or pressure on top of a single wire, or a group ofwires, such that these wires transfer to transfer substrate 2106, butadditional wires, perhaps oriented in another direction, are notcontacted and do not transfer. In embodiments where a heated probe isused, as the transfer substrate cools, the nanowires will remainattached to and/or embedded in the transfer substrate. In embodimentswhere an adhesive coats the surface of the transfer substrate, uponremoval of the pressure applied by probe 2108, the nanowires will remainattached to the transfer substrate via the adhesion between thenanowires and the adhesive.

In other embodiments of the present invention, substrate 2102 can beheated, rather than, or in addition to, probe 2108 and probe tip 2110being heating. In such embodiments, substrate 2102 can serve as theheat-generating portion of transfer scheme 2100, and pressure applied byprobe 2108 allows for conductive heating of transfer substrate 2106 sothat nanowires 2104 will transfer and remain attached to transfersubstrate 2106.

The processes of the present invention can be used to transfer nanowiresto select regions of transfer substrate 2106. Only regions where contactis made between transfer substrate 2106 and nanowires 2104, will thenanowires be transferred. Such embodiments of the present invention arereferred to herein as a “tapping” method of nanowire transfer. In suchembodiments, the probe tip can be moved around the transfer substrate,“tapping” the nanowires below to facilitate transfer from the substrate2102 to the transfer substrate 2106. In other embodiments, the probe canbe held stationary and the substrate and transfer substrate movedbeneath it so as to control where nanowire transfer occurs, and theorientation of the nanowires on the transfer substrate. As discussedabove, in such embodiments, either, or both, substrate 2102 and probe2108/probe tip 2110 can be heated. Such embodiments of the presentinvention allow orientation of nanowires directly on transfer substrate2106 by selectively transferring wires that have already been orientedon substrate 2102 using such methods as described herein (e.g., LangmuirBlodget, e-field, epitaxial growth, horizontal growth, etc.), ororienting the wires on the transfer substrate 2106 can also be achievedby transferring individual wires, or groupings of wires, and positioning(e.g., rotating) the transfer substrate 2106 such that the wires areoriented on the transfer substrate as they are transferred.

FIG. 22 is a representation of global nanowire transfer in accordancewith one embodiment of the present invention. In other embodiments ofthe present invention, as illustrated in FIG. 22, nanowire transfer fromsubstrate 2102 to transfer substrate 2106 can be achieved by applyingsubstantially uniform pressure over a larger area of the substrate, suchas with a large area compressive device 2202, or a vacuum. As usedherein, substantially uniform pressure indicates that the pressureapplied to greater than about 50% of the total area of transfersubstrate 2106 is the same. Suitably, about 90-100% of the total area oftransfer substrate 2106 will have the same pressure applied across it.In certain such embodiments, the surface of transfer substrate 2106 cancomprise an adhesive layer such that nanowires that come in contact withthe adhesive layer will attach and remain attached. In other embodimentsof the present invention, substrate 2102 can be heated, therebyconductively heating transfer substrate 2106, which aids in the transferof nanowires 2104 to a now tacky or sticky transfer substrate 2106. Insuch embodiments, a global transfer of nanowires 2104 from substrate2102 to transfer substrate 2106 occurs, and a substantial portion of thenanowires are transferred to the transfer substrate.

In embodiments of the present invention where a global transfer of wiresis desired, pressure can be applied between substrate 2102 and transfersubstrate 2106 by applying a vacuum. In such embodiments, a vacuum canbe generated between transfer substrate 2106 and substrate 2102 suchthat there is a substantially uniform pressure over the entire transfersubstrate 2106, allowing nanowire transfer at substantially all contactpoints between transfer substrate 2106 and nanowires 2104. Suitablevacuum pressures can be readily determined by those skilled in the art,and will generally be in the range of 10's of psi, suitably about 40 psito about 100 psi.

In embodiments of the present invention where this global transfertechnique is used, nanowires 2104 can first be pre-aligned on substrate2102 prior to transfer to transfer substrate 2106. Any suitable nanowirealignment process can be used. In certain embodiments, the nanowireswill be pre-aligned on substrate 2102 using electric field (e-field)alignment. FIGS. 23A-C illustrate transfer of e-field aligned wires.FIG. 23A illustrates an e-field alignment of nanowires prior totransfer. FIG. 23B illustrates nanowires 2104 remaining on substrate2102 after global transfer. FIG. 23C illustrates nanowires transferredto transfer substrate 2106. One additional advantage of the globaltransfer technique, and the probe technique described above, is thatsubstrate 2102 can be used repeatedly for nanowire growth and transferafter the nanowires that have grown on its surface have been transferredto a transfer substrate.

Transfer substrate 2106 utilized in any embodiments of the presentinvention can also have various device contacts deposited on its surfaceeither prior to, or after nanowire transfer. For example, as describedherein, source, drain and gait electrodes can be added to transfersubstrate 2106, and then nanowires transferred to specific areas oftransfer substrate 2106 either using the probe transfer process 2100 orglobal transfer process 2200 described herein. In embodiments where theglobal transfer processes is used, wires will suitably be aligned priorto transfer such that the device can be assembled directly on thetransfer substrate.

Use of Nanowires of the Present Invention in Exemplary Devices andApplications

Numerous electronic devices and systems can incorporate semiconductor orother type devices with thin films of nanowires produced by the methodsof the present invention. Some example applications for the presentinvention are described below or elsewhere herein for illustrativepurposes, and are not limiting. The applications described herein caninclude aligned or non-aligned thin films of nanowires, and can includecomposite or non-composite thin films of nanowires.

Semiconductor devices (or other type devices) can be coupled to signalsof other electronic circuits, and/or can be integrated with otherelectronic circuits. Semiconductor devices can be formed on largesubstrates, which can be subsequently separated or diced into smallersubstrates. Furthermore, on large substrates (i.e., substratessubstantially larger than conventional semiconductor wafers),semiconductor devices formed thereon can be interconnected.

The nanowires produced by the processes of the present invention canalso be incorporated in applications requiring a single semiconductordevice, and to multiple semiconductor devices. For example, thenanowires produced by the processes of the present invention areparticularly applicable to large area, macro electronic substrates onwhich a plurality of semiconductor devices is formed. Such electronicdevices can include display driving circuits for active matrix liquidcrystal displays (LCDs), organic LED displays, field emission displays.Other active displays can be formed from a nanowire-polymer, quantumdots-polymer composite (the composite can function both as the emitterand active driving matrix). The nanowires produced by the processes ofthe present invention are also applicable to smart libraries, creditcards, large area array sensors, and radio-frequency identification(RFID) tags, including smart cards, smart inventory tags, and the like.

The nanowires produced by the processes of the present invention arealso applicable to digital and analog circuit applications. Inparticular, the nanowires produced by the processes of the presentinvention are useful in applications that require ultra large-scaleintegration on a large area substrate. For example, a thin film ofnanowires produced by the processes of the present invention can beimplemented in logic circuits, memory circuits, processors, amplifiers,and other digital and analog circuits.

The nanowires produced by the processes of the present invention can beapplied to photovoltaic applications. In such applications, a clearconducting substrate is used to enhance the photovoltaic properties ofthe particular photovoltaic device. For example, such a clear conductingsubstrate can be used as a flexible, large-area replacement for indiumtin oxide (ITO) or the like. A substrate can be coated with a thin filmof nanowires that is formed to have a large bandgap, i.e., greater thanvisible light so that it would be non-absorbing, but would be formed tohave either the HOMO or LUMO bands aligned with the active material of aphotovoltaic device that would be formed on top of it. Clear conductorscan be located on two sides of the absorbing photovoltaic material tocarry away current from the photovoltaic device. Two different nanowirematerials can be chosen, one having the HOMO aligned with that of thephotovoltaic material HOMO band, and the other having the LUMO alignedwith the LUMO band of the photovoltaic material. The bandgaps of the twonanowires materials can be chosen to be much larger than that of thephotovoltaic material. The nanowires, according to this embodiment, canbe lightly doped to decrease the resistance of the thin films ofnanowires, while permitting the substrate to remain mostlynon-absorbing.

Hence, a wide range of military and consumer goods can incorporate thenanowires produced by the processes of the present invention. Forexample, such goods can include personal computers, workstations,servers, networking devices, handheld electronic devices such as PDAsand palm pilots, telephones (e.g., cellular and standard), radios,televisions, electronic games and game systems, home security systems,automobiles, aircraft, boats, other household and commercial appliances,and the like.

Oriented Growth of Nanowires on Patterned Substrates

In another embodiment, the present invention provides methods forproducing nanowires on patterned substrates. FIG. 24 shows a diagram oforiented nanowire growth using a patterned substrate in accordance withone embodiment of the present invention. It should be noted that FIG. 24is not to scale and is simply provided to illustrate certain aspects oforiented nanowire growth on patterned substrates in accordance with thisembodiment of the present invention. FIG. 25 shows a flowchart 2500describing a method for producing nanowires utilizing patternedsubstrates in accordance with one embodiment of the present invention.

As shown in FIG. 24, a substrate material 2402, suitably acrystallographic substrate, such a silicon or other semiconductormaterial, is provided. In step 2502 of flowchart 2500, acatalyst-repelling material 2404 is applied on substrate material 2402to at least partially cover the substrate material 2402. As used herein,the terms “applying” or “applied” refers to any suitable method forpreparing a catalyst-repelling material 2404 on a substrate material2402, and includes, layering depositing, spraying, coating, etc. As usedherein the phrase “at least partially cover the substrate material”means that the catalyst-repelling material 2404 covers at least 1% ofthe surface area of substrate material 2404. Suitably, thecatalyst-repelling material 2404 will cover at least about 10%, at leastabout 20%, at least about 30%, at least about 40%, at least about 50%,at least about 60%, at least about 70%, at least about 75%, at leastabout 80%, at least about 90%, or at least about 95% of the surface areaof substrate material 2402.

As used herein the term “catalyst-repelling material” includes anymaterial which does not allow a catalyst material to substantially bond,affix, interact, or attach thereto. Catalyst-repelling materials for usein the present invention suitably do not allow nucleating catalystparticles to bond or attach to their surface, thereby creating arepelling effect whereby nucleating particles do not, or cannot, attach,and as such are driven to areas of the substrate material which are notcovered by the catalyst-repelling material. In other embodiments,nucleating catalyst particles are driven from the catalyst repellingmaterial, or leave, for example during application (evaporation) orheating of the catalyst material. Examples of catalyst-repellingmaterials include, but are not limited to SiO₂ and anodic alumina. Asshown in FIG. 24, in suitable embodiments, catalyst-repelling material2404 will be patterned or otherwise prepared such that the material atleast partially covers substrate material 2402.

In suitable embodiments, catalyst-repelling material 2404 comprisesvoids 2410 in the material that expose substrate material 2402 below.The term “voids” as used herein includes, holes, openings, cracks, orother patterns that expose portions of substrate material 2402, whilecontinuing to at least partially cover substrate material 2402.Suitably, voids 2410 are spaced throughout the catalyst-repellingmaterial 2404 such that when nanowires are grown at the sites of thevoids, they are spaced far enough from each other such that they don'tcontact or disturb other growing nanowires. Any suitable orientation ofvoids 2410 can be used. In addition, voids 2410 can be any desirableshape, for example, circular, square, random, etc. In suitableembodiments, voids 2410 are randomly spaced across catalyst-repellingmaterial 2404 to create a variety of shapes and spacings. In otherembodiments, voids 2410 can be evenly spaced, or in an oriented pattern,throughout catalyst-repelling material 2404, for example as acheckerboard, or other application-specific pattern. Voids 2410 caneither be created in catalyst repelling material 2404 simply by formingthem during initial application (i.e., by forming them around a voidopening or “mold”), or they can be generated, for example as shown instep 2514, in flowchart 2500 of FIG. 25. For example, voids 2410 can begenerated in step 2514 by removing catalyst-repelling material 2404 toexpose substrate material 2402 via any suitable method, for example, byetching, cutting, scraping, drilling, or similar method.Catalyst-repelling material 2404 can be any suitable thickness that willprevent nucleating particles from contacting the substrate surface inareas which are desired to be covered. Suitably, catalyst-repellingmaterial 2404 will be on the order of several nanometers to a fewmicrons in thickness, though thicker material can also be used.

In step 2504 of flowchart 2500, one or more nucleating particles 2406are then applied to the substrate material 2402. When nucleatingparticles 2406 contact catalyst-repelling material 2404, the particlesare repelled and move to open voids 2410 where substrate material 2402is exposed, or leave the substrate all together (e.g., during anevaporation deposition process). Nucleating particles that initiallywere in contact with the substrate material following application (i.e.,contact at the sites of the voids) remain on the substrate material. Asdescribed herein, nucleating particles are suitably metallic catalyststhat react with the decomposed precursor gas mixture(s) to form aeutectic from which Si precipitates, such as described throughout. Forexample, Au, Al, Pt, Fe, Ti, Ga, Ni, Sn or In can be deposited.Nucleating particles 2406 can be deposited as colloid droplets directlyon the surface of substrate surface 2404. In additional embodiments,colloid droplets can be deposited on catalyst-repelling material 2404,at which time, assuming they are in a liquid state, they will move backinto solution and may subsequently migrate from the catalyst-repellingmaterial to the voids 2410 where they will contact substrate material2402.

In additional embodiments, nucleating particles comprise metallic films,such as gold (Au), aluminum (Al) or other metal films as describedthroughout. Depositing a metallic film on catalyst-repelling material2404 causes the metallic catalyst to migrate to open voids 2410 in thematerial where it enters the voids and then contacts substrate material2402, or leave the substrate all together (e.g., during an evaporationdeposition process). Nucleating particles that initially were in contactwith the substrate material following application (i.e., contact at thesites of the voids) remain on the substrate material. In cases of bothmetallic colloids and metallic films, the catalyst material migrates toopen areas of the patterned catalyst-repelling material 2404, or leavesthe catalyst-repelling material, assuming that the catalyst is in aliquid state. Catalyst material can either be deposited in a liquidstate, or can be deposited in a solid state and then, as shown in step2512 of flowchart 2500, heated to a temperature where it melts and canflow/migrate or leave/evaporate. At this elevated temperature, metalliccatalyst that is in contact with catalyst-repelling material 2404 canmigrate to open voids 2410 and contact substrate material 2402.Alternatively, at the elevated temperature, the metallic catalyst incontact with the catalyst-repelling material leaves (e.g., evaporates)while the metallic catalyst that is in contact with the underlyingsubstrate (i.e., in the void) coalesces into a metallic particle thatcan be alloyed with the substrate. Migration and/or coalescence of thenucleating particles can occur just prior to, or at the beginning, ofthe nanowire growth process.

In suitable embodiments, nucleating particles 2406 are applied/depositedonto catalyst-repelling material 2404 and substrate material 2402 byevaporating a metallic film (e.g., Al, Au or other suitable material)onto the substrate. Suitably, application of the nucleating particles(e.g., by evaporating a metallic film) occurs at room temperature (e.g.,about 20-28° C.) and at a reduced pressure (i.e., in a vacuum at apressure of less than about 10⁻⁷ torr, for example, between about 5*10⁻⁸to about 10⁻⁷ torr). In further embodiments, application of thenucleating particles (e.g., via film evaporation) can be performed at anelevated temperature and reduced pressure. For example, the film can beevaporated at a temperature of greater than about 600° C., e.g., greaterthan about 650° C., greater than about 700° C., greater than about 750°C., greater than about 760° C., greater than about 770° C., about 775°C., about 780° C., about 790° C., or about 800° C.

The thickness of the metallic film deposited on the catalyst-repellingmaterial 2404 and substrate 2402 will suitably be on the order of a fewnanometers to 10's of nanometers thick, e.g., about 3-50 nm thick,suitably about 5-10 nm thick. Following application of the film (e.g.,evaporation to form the metallic film), the film is then heated (e.g.,step 2512 in flowchart 2500) to remove it from the catalyst-repellingmaterial 2404, either via evaporation and/or by flowing into the voidsin catalyst-repelling material 2404, thereby allowing the nucleatingparticles to coalesce and alloy with the substrate 2402. Suitably, thefilm is heated to between about 500° C. to about 900° C. to cause it toflow into voids 2410. For example, the film is suitably heated tobetween about 600° C. to about 800° C., to between about 650° C. toabout 800° C., to between about 700° C. to about 800° C., about 725° C.,about 750° C., about 760° C., about 770° C., about 775° C., about 780°C., about 790° C., or about 800° C. In suitable embodiments, heatingstep 2512 occurs at a reduced pressure, for example, between about5*10⁻⁸ to about 10⁻⁷ torr. Following heating of the film, the substrateis then suitably cooled, and then transferred to a CVD reactor or othersuitable apparatus to grow nanowires. Generally, the application of thenucleating particles (e.g., deposition of nanoparticles or evaporationof a catalyst film, followed by subsequent heating) and the contactingof the nanowires with a precursor gas mixture (i.e., growth) occur inseparate reaction chambers, though they can occur in the same chamber.Suitably, the application of the nucleating particles occurs in an highvacuum chamber, while the growth occurs in a separate CVD reactor.

Once the nucleating particles 2406 are deposited on substrate material2402, either directly, following evaporation from the catalyst-repellingmaterial, or after migrating from catalyst-repelling material 2404, thenucleating particles 2406 are heated in step 2506 and contacted with aprecursor gas mixture in step 2508 of flowchart 2500 (e.g., in a CVDreactor) to create a liquid alloy droplet 2412, whereby nanowire 2408growth occurs at the site of the liquid alloy droplet in step 2510.Suitable growth conditions, including temperatures and times, aredescribed herein. Suitable precursor gases include those describedherein, and include gases comprising, but not limited to, SiH₄, SiCl₄and SiH₂Cl₂. The use of catalyst-repelling material 2404, in addition toaiding in deposition of nucleating particles, also helps to keepnucleating particles 2406 from migrating during nanowire growth. Ifnucleating particles 2406 are heated in the absence ofcatalyst-repelling material 2404, they can often migrate on substratematerial 2402 and coalesce into larger nucleating particles. This cancompromise nanowire diameter and structure. In addition to problems withcoalescence, catalyst-repelling material 2404 also helps to keepnucleating particles 2406 and growing nanowires 2408 properly spaced andoriented, thereby reducing tangling and other complications.

As shown in flowchart 2500 of FIG. 25, suitably, a single precursor gasis used to grow nanowires. In further embodiments, a second precursorgas can be utilized, as described throughout. For example, followingcontacting with a first precursor gas, alloy droplet 2412 can be heatedto a second temperature, and contacted with a second precursor gasmixture, to continue nanowire growth at the site of the alloy droplet.Suitable gases for use as the second precursor gas include thosedescribed herein, including, but not limited to, SiH₄, Si₂H₆, SiCl₄ andSiH₂Cl₂. In further embodiments, a third, fourth, fifth, etc., precursorgas mixture can be provided to continue growing the nanowire(s). In suchembodiments, the temperature of the growing nanowires 2408 and alloydroplets 2412 is maintained at a suitable temperature to allow precursorgas dissociation and nanowire growth. Thus, in suitable embodiments, thepresent invention provides for a continuously varying growth process inwhich the temperature of the nanowire growth and the precursor gasesused can be continuously switched throughout the growth process untilthe final nanowire composition and characteristics (i.e., length,diameter) are achieved. Suitably, the first temperature used in thenanowire growth methods will be higher than the second temperature, forexample about 50° C. higher. Any suitable method can be used tointroduce the gases for the nanowire growth process. For example, plasmaenhanced sputter deposition can be used to introduce the precursor gasmixtures. Rapid control of the chamber and substrate chamber can beachieved through any method known in the art. For example, a cold wallin an ultra high vacuum (UHV) reactor can be used.

In embodiments in which a single precursor gas mixture is utilized togrow nanowires (as well as where two or more precursor gas mixtures areutilized), the pressure and temperature of the contacting/growthconditions will suitably be greater than about 400° C., for example,between about 450° C. to about 700° C., and greater than about 0.5 torr,for example, between about 5 torr to about 200 torr. For example,suitable growth temperatures for use in the practice of the presentinvention are between about 475° C. to about 675° C., about 500° C. toabout 650° C., about 550° C. to about 650° C., about 575° C. to about625° C., or about 580° C., about 590° C., about 600° C., about 610° C.,or about 620° C. Suitable precursor gas mixture pressures for use in thepractice of the present invention are between about 5 torr to about 175torr, about 10 torr to about 150 torr, about 20 torr to about 150 torr,about 40 torr to about 150 torr, about 45 torr, about 50 torr, about 55torr, about 60 torr, about 65 torr, about 70 torr, about 75 torr, about80 torr, about 85 torr, about 90 torr, about 95 torr, about 100 torr,about 105 torr, about 110 torr, about 115 torr or about 120 torr.

As described throughout, the present invention also provides nanowiresproduced by such processes of the present invention, and electroniccircuits comprising such nanowires.

By combining patterned substrates and in suitable embodiments, varyingtemperature/precursor gas growth conditions, the nanowires producedaccording to the methods of the present invention are substantiallyvertical, oriented nanowires. Suitably, the methods produce epitaxiallyoriented nanowires that grow substantially normal to the plane of thesubstrate material. By controlling the deposition of nucleatingparticles, thereby controlling their migration on the substratematerial, nanowire thickness is controlled. The use of a singleprecursor gas mixture (e.g., a single temperature and pressure) as wellas use of varied growth conditions (two or more precursor gas mixturesat different temperatures and pressures) substantially aid in producingnanowires that do not exhibit taper throughout their length, but rathershow substantial uniformity throughout. Suitably, nanowires producedusing the various methods of the present invention exhibit a degree oftaper that is less than about 0.1 nm taper/μm nanowire length. Infurther embodiments, nanowire taper can also be controlled, eliminatedor substantially eliminated by introducing an etchant gas into thereaction prior to, during, or after nanowire growth. For example, asdisclosed in U.S. Provisional Patent Application No. 60/857,450, filedNov. 7, 2006 (the disclosure of which is incorporated herein byreference in its entirety), HCl can be introduced to control, eliminateor substantially eliminate nanowire tapering during growth.

In further embodiments, the present invention provides methods forproducing nanowires. Suitably, a catalyst-repelling material is appliedon a silicon substrate, to at least partially cover the siliconsubstrate, wherein the catalyst-repelling material comprises at leastone void that does not cover the silicon substrate. One or more metallicnucleating particles are then applied on the catalyst-repellingmaterial, wherein the metallic nucleating particles deposit and coalesceon the silicon substrate, via selective evaporation from thecatalyst-repelling material and/or migration to the at least one void.The metallic nucleating particles are then heated to a first temperatureand contacted with a first precursor gas mixture to create a liquidalloy droplet to initiate nanowire growth. The alloy droplet is thenheated to a second temperature and contacted with a second precursor gasmixture, whereby nanowires are grown at the site of the alloy droplet.Suitably the nucleating particles will be metallic catalysts, such asmetallic films or colloids (e.g., Au or Al films or colloids). The useof crystallographic substrates, such as Si <111> substrates (as well asother crystal orientations and substrate materials), allows forsubstantially oriented (suitable epitaxially oriented), verticalnanowires with substantially constant diameter and little taper. Asdescribed herein, suitably the catalyst-repelling material comprisesSiO₂, or anodic alumina. Examples of precursor gases include thosedescribed herein, such as SiH₄, SiCl₄ and SiH₂Cl₂. In additionalembodiments, further precursor gas compositions and conditions can beused to continuously vary the growth process.

Additional methods for producing nanowires are also provided. Forexample, in and additional embodiment, a catalyst-repelling material isapplied on a silicon substrate to at least partially cover the siliconsubstrate, wherein the catalyst-repelling material comprises at leastone void that does not cover the silicon substrate. One or more metallicnucleating particles are then applied on the catalyst-repellingmaterial, wherein the metallic nucleating particles deposit and coalesceon the silicon substrate, via selective evaporation from thecatalyst-repelling material and/or migration to the at least one void.The metallic nucleating particles are then heated (e.g., to atemperature of greater than about 400° C.). The metallic nucleatingparticles are then contacted with a precursor gas mixture (e.g., at apressure greater than about 0.5 torr) to create an alloy droplet,whereby nanowires are grown at the site of the alloy droplet.

Suitable conditions, including temperature and pressure conditions, forapplying nucleating particles are described throughout. For example, theapplication process can comprise evaporating a metallic film either atroom temperature (e.g., about 20° C.-28° C.), or at an elevatedtemperature (e.g., greater than about 600° C.) and at a reducedpressure, for example, between about 5*10⁻⁸ to about 10⁻⁷ torr. Suitablythe application of the nucleating particles and the contacting with aprecursor gas mixture occur in different reaction chambers.

Exemplary catalyst repelling materials, nucleating particles andprecursor gas mixtures are described throughout. Suitably, the heatingis to a temperature of about 450° C. to about 700° C., and the pressureis between about 5 torr and about 200 torr, suitably about 45 torr. Insuitable embodiments, the step of applying the nucleating particlesfurther comprises heating to a temperature wherein the metallic filmmelts (e.g., about 450-900° C.), thereby generating more metallicnucleating particles on the catalyst-repelling material, wherein themetallic nucleating particles deposit and coalesce on the substrate, viaselective evaporation from the catalyst-repelling material and/ormigration to the at least one void.

In still further embodiments, the present invention provides methods forproducing nanowires utilizing a single precursor gas mixture. Forexample as shown in flowchart 2600 in FIG. 26, in step 2602, one or morenucleating particles is applied on a substrate material. In step 2604,the nucleating particles are then heated to a temperature of greaterthan about 550° C. In step 2606, the nucleating particles are thencontacted with a precursor gas mixture at a pressure of greater thanabout 0.5 torr to create a liquid alloy droplet, whereby nanowires growat the site of the alloy droplet in step 2608.

Exemplary substrate materials (e.g, Si), nucleating particles (e.g., Auor Al) and precursor gas mixtures (e.g., SiH₄) are described throughout.Suitably, the nucleating particles are heated to a temperature of about600° C. to about 700° C., e.g., about 600° C., and then contacted with aprecursor gas mixture at between about 5 torr to about 200 torr,suitably about 45 torr. As discussed throughout, the application processcan comprise evaporating a metallic film either at room temperature(e.g., about 20° C.-28° C.), or at an elevated temperature (e.g.,greater than about 600° C.) and at a reduced pressure, for example,between about 5*10⁻⁸ to about 10⁻⁷ torr. Suitably the application of thenucleating particles and the contacting with a precursor gas mixtureoccur in different reaction chambers.

CONCLUSION

Exemplary embodiments of the present invention have been presented. Theinvention is not limited to these examples. These examples are presentedherein for purposes of illustration, and not limitation. Alternatives(including equivalents, extensions, variations, deviations, etc., ofthose described herein) will be apparent to persons skilled in therelevant art(s) based on the teachings contained herein. Suchalternatives fall within the scope and spirit of the invention.

All publications, patents and patent applications mentioned in thisspecification are indicative of the level of skill of those skilled inthe art to which this invention pertains, and are herein incorporated byreference to the same extent as if each individual publication, patentor patent application was specifically and individually indicated to beincorporated by reference.

1. A method for producing nanowires, comprising: (a) creating orgenerating at least one void in a catalyst-repelling material on asubstrate material; (b) depositing a metallic film directly on thecatalyst-repelling material and the substrate material at the site ofthe at least one void; (c) heating the metallic film to a temperaturewherein the metallic film melts, thereby generating one or more metallicnucleating particles, wherein the nucleating particles migrate from thecatalyst-repelling material to the site of the at least one void anddeposit on the substrate; and (d) heating the nucleating particles to atemperature of greater than about 400° C. and contacting the nucleatingparticles with a precursor gas mixture at a pressure greater than about0.5 torr to create an alloy droplet, whereby nanowires are grown at thesite of the alloy droplet.
 2. The method of claim 1, wherein thecreating or generating in (a) comprises layering a catalyst-repellingmaterial on a crystallographic substrate material.
 3. The method ofclaim 2, wherein the substrate comprises silicon, and the creating orgenerating in (a) comprises layering a catalyst-repelling material onthe silicon substrate.
 4. The method of claim 2, wherein the growingoccurs epitaxially.
 5. The method of claim 1, wherein the creating orgenerating in (a) comprises layering a catalyst-repelling material whichcomprises SiO₂, or anodic alumina.
 6. The method of claim 1, wherein thegrowing is out of a plane of the substrate material.
 7. The method ofclaim 1, wherein the heating in (d) is to a temperature of about 450° C.to about 700° C.
 8. The method of claim 1, wherein the depositing in (b)comprises depositing a metallic film comprising a metal that reacts withthe precursor gas mixture of step (d) to form a eutectic from which Siprecipitates.
 9. The method of claim 8, wherein the depositing in (b)comprises depositing a Au, Al, Pt, Fe, Ti, Ga, Ni, Sn or In film. 10.The method of claim 9, wherein the depositing in (b) comprisesdepositing a Au or Al film.
 11. The method of claim 1, wherein thecontacting with a precursor gas mixture comprises contacting with a gasmixture comprising SiH₄, Si₂H₆, SiCl₄ or SiH₂Cl₂.
 12. The method ofclaim 1, wherein the contacting comprises performing plasma enhancedsputter deposition.
 13. The method of claim 1, wherein the contactingoccurs at a pressure of between about 5 to about 200 torr.
 14. Themethod of claim 13, wherein the contacting occurs at about 45 torr. 15.The method of claim 1, wherein the depositing in (b), and the contactingin (d) occur in separate reaction chambers.
 16. Nanowires produced bythe process of claim
 1. 17. An electronic circuit comprising nanowiresproduced by the process of claim
 1. 18. The method of claim 1, whereinthe creating or generating in (a) comprises creating or generating voidsin an oriented pattern.
 19. The method of claim 1, wherein thedepositing in (b) comprises depositing a 3-50 nm thick metallic film.20. The method of claim 1, wherein the heating in (c) comprises heatingto a temperature of about 500° C. to about 900° C. at a pressure ofabout 5*10⁻⁸ to about 10⁻⁷ torr.
 21. A method for producing nanowires,comprising: (a) creating or generating at least one void in acatalyst-repelling material on a silicon substrate; (b) depositing ametallic film directly on the catalyst-repelling material and thesilicon substrate at the site of the at least one void; (c) heating themetallic film to a temperature wherein the metallic film melts, therebygenerating one or more metallic nucleating particles, wherein thenucleating particles migrate from the catalyst-repelling material to thesite of the at least one void and deposit on the silicon substrate; and(d) heating the metallic nucleating particles to a temperature ofgreater than about 400° C. and contacting the metallic nucleatingparticles with a precursor gas mixture at a pressure greater than about0.5 torr to create an alloy droplet, whereby nanowires are grown at thesite of the alloy droplet.
 22. The method of claim 21, wherein thecreating or generating in (a) comprises layering a catalyst-repellingmaterial which comprises SiO₂, or anodic alumina.
 23. The method ofclaim 21, wherein the growing occurs epitaxially.
 24. The method ofclaim 21, wherein the growing is out of a plane of the siliconsubstrate.
 25. The method of claim 21, wherein the heating in (d) is toa temperature of about 450° C. to about 700° C.
 26. The method of claim21, wherein the depositing in (b) comprises depositing a Au, Al, Pt, Fe,Ti, Ga, Ni, Sn or In film.
 27. The method of claim 26, wherein thedepositing in step (b) comprises depositing a Au or Al film.
 28. Themethod of claim 21, wherein the contacting with a precursor gas mixturecomprises contacting with SiH₄, Si₂H₆, SiCl₄ or SiH₂Cl₂.
 29. The methodof claim 21, wherein the contacting comprises performing plasma enhancedsputter deposition.
 30. The method of claim 21, wherein the contactingoccurs at a pressure of about 5 to about 200 torr.
 31. The method ofclaim 30, wherein the contacting occurs at about 45 torr.
 32. The methodof claim 21, wherein the depositing in (b), and the contacting in (d)occur in separate reaction chambers.
 33. Nanowires produced by theprocess of claim
 21. 34. An electronic circuit comprising nanowiresproduced by the process of claim
 21. 35. The method of claim 21, whereinthe creating or generating in (a) comprises creating or generating voidsin an oriented pattern.
 36. The method of claim 21, wherein thedepositing in (b) comprises depositing a 3-50 nm thick metallic film.37. The method of claim 21, wherein the heating in (c) comprises heatingto a temperature of about 500° C. to about 900° C. at a pressure ofabout 5*10⁻⁸ to about 10⁻⁷ torr.